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Operational characteristics of an MEMS-based micro oscillating heat pipe
Applied Thermal Engineering 124 (2017) 1269–1278
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Operational characteristics of an MEMS-based micro oscillating heat pipe Qin Sun a, Jian Qu b,⇑, Jianping Yuan a, Qian Wang b a b
Research Center of Fluid Machinery Engineering and Technology, Jiangsu University, Zhenjiang 212013, China School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China
h i g h l i g h t s The thermo-fluidic characteristics of an MEMS-based micro-OHP were experimentally investigated. The micro-OHP worked well at a Bond number less than 0.4 and the gravity action still functioned. Alternation of small- and large-amplitude oscillation phase was observed in the micro-OHP. 2
The micro-OHP could sustain a maximum heat flux up to about 20.3 W/cm .
a r t i c l e
i n f o
Article history: Received 13 February 2017 Revised 18 May 2017 Accepted 21 June 2017 Available online 22 June 2017 Keywords: Micro oscillation heat pipe Thermal performance Start-up Gravity effect Dry-out
a b s t r a c t The flow characteristics and thermal performance of an MEMS-based micro oscillating heat pipe (microOHP) were experimentally investigated using dielectric liquid HFE-7100 as working fluid. The micro-OHP was integrated on a silicon wafer with trapezoidal channels having a hydraulic diameter of 357 lm. The gravity action on the micro-OHP performance functioned greatly and could not be ignored. High-speed visual observation demonstrated the annular/semi-annular flow, injection/slug flow, and bubbly flow in the micro-OHP. The alternate ‘‘small-amplitude oscillation phase (SAOP)—large-amplitude oscillation phase (LAOP)” was verified in this micro-OHP at the quasi-steady oscillation state, and its performance improvement was attributable to the extended time ratio of LAOP. In addition, the ephemeral circulation flow and bubble nucleation were observed intermittently. The occurrence of dry-out mainly appeared at lower filling ratios and higher power inputs. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction The goal of enhancing heat transfer while reducing the size and volume of energy conversion/thermal management systems has been the subject of intensive research for several decades. However, challenges such as growing energy demands, the need for increased energy efficiency and material savings, increased functionality and ease of unit handling as well as space limitations for device packaging have led to advanced research into the development of compact heat exchangers, mainly for the thermal management of high performance electronic devices. To provide advanced thermal control solution for nextgeneration electronic devices, persistent efforts have been made to develop novel micro cooling technologies capable of removing large amount of heat from chips or improving temperature
⇑ Corresponding author. E-mail address: [email protected] (J. Qu). http://dx.doi.org/10.1016/j.applthermaleng.2017.06.109 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
uniformity [1–4]. Among them, oscillating heat pipe (OHP), a high-efficiency two phase heat transfer device invented by Akachi [5], is considered as a promising solution and has been attracted considerable attention recent years [6–13] due to the simple structure, high performance, and acceptable reliability. Traditionally, OHPs are made from serpentine-arranged, interconnected capillary tubes. Although the OHP structure is quite simple, the thermo-fluidic operational characteristic is highly non-linear and extremely complex at the capillary level [14,15]. In an OHP, the bubble growth and expansion at the hot region and simultaneous bubble shrinking and collapse at the cold region provide a pumping action to transport entrapped liquid slugs in a complex pulsating–translating–vibratory fashion [16], resulting in self-sustained thermally driven flow oscillations. In addition to the latent heat, considerable amount of sensible heat transfer also occurs in the OHP, i.e., Taylor bubble train flow at pulsating condition and the resultant slug-plug distribution contribute significantly to the net heat transport [17]. To reveal the operational
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Nomenclature Bo cp D es eR g hfg I L m N Q R t T U
Bond number specific heat (J/(kgK)) hydraulic diameter (m) system uncertainty random uncertainty gravitational acceleration (m/s2) vaporization latent heat (kJ/kg) input current (A) length (m) mass flow rate (kg/s) number of LAOP behavior heating power (W) thermal resistance (°C/K) time (s) temperature (°C) input voltage (V)
mechanism of OHPs, some studies on flow visualization mainly using transparent glass OHPs have been conducted. Tong et al. [6] performed the flow visualization for a closed loop glass OHP filled with 60% methanol. It was observed that the working fluid oscillates with large amplitude during the start-up process, however, at the quasi-steady state, the working fluid circulated and coupled with complex process such as nucleation boiling and coalescence of bubbles. Bulk circulation flow and local flow direction switch in a multi-turn OHP were also observed and verified by Xu et al. [9]. In addition, they found that both of the bubble displacement and velocity displayed quasi-sine oscillating waves in the methanol-charged OHP, while the water-charged OHP had quasi-rectangular shape for bubble displacements. Senjaya and Inoue [18] investigated the boiling phenomena in the evaporator section of an OHP, and found that the sudden and acceleration motion of liquid-vapor interfaces is mainly triggered by the tubesized bubble generation and growth, which also provides the large driving force to support the random slug/plug motions. To further understand the internal thermo-hydrodynamic phenomena in OHPs, some other efforts have been made to explore the thermofluidic characteristics of OHPs [17,19,20]. Due to the wickless style, OHPs are suitable for miniaturization and could be fabricated and integrated directly into semiconductor chips on the basis of micro-electromechanical system (MEMS) technology, acting as thermal spreaders. In addition, our recent theoretical analysis [21] demonstrated that the low limit of internal diameter for an OHP could be less than 0.3 mm for its operation when proper working fluids are selected. In recent years, tenacious efforts, mainly experimentally, have also been made to understand the internal thermo-hydrodynamic behavior of micro-OHPs. Qu et al. [22–24] tentatively designed and fabricated several microOHPs on silicon substrates with trapezoidal channels, characterized by hydraulic diameters ranging from 251 to 393 lm. Experimental tests demonstrated that these micro devices could operate normally with sustained oscillatory flow and function satisfactorily. At an allowable temperature of 109 °C, an overall rather than local heat flux up to 10.7 W/cm2 was achieved for the FC-72 charged micro-OHP with each channel having a hydraulic diameter of 352 lm [23]. To understand the complex oscillating flow in micro-OHPs, Yoon and Kim [25] conducted a flow visualization experiment and proposed a vapor spring-liquid mass model to explain the oscillating mechanisms. Inspired by non-uniform channel configuration OHP design applicable to all orientations proposed by Wang and his co-workers [26,27], non-uniform or
Greek
r q
k
g
surface tension (N/m) density (kg/m3) thermal conductivity (W/(mK)) dynamic viscosity (kg/(ms))
Subscript a adiabatic section c condenser section e evaporator section in inlet l liquid out outlet s saturation v vapor 1, 2, . . .,8 thermocouple number
alternate channel has recently been developed by Yang et al. [28,29] to facilitate the start-up and improve the micro-OHP performance. Kwon and Kim [30] also conducted an experimental study to investigate the effect of dual-diameter (or alternate) channel on the flow and heat transfer characteristics of silicon-based micro-OHPs. Experimental results demonstrated that the microOHPs with dual-diameter channels have better thermal performance and can operate independently irrespective of the orientation as compared to the micro-OHP with uniform channels. However, the underlying thermo-fluidic behavior is quite complex at the micro-scale level and yet to be fully revealed, requiring further experimental investigations to shed light on the fluid flow and heat transfer characteristics to provide a deep understanding of it. In this paper, the flow behavior, slug/plug oscillation characteristics, and flow pattern evolution subject to different heat input levels in an MEMS-based micro-OHP fabricated on silicon wafer were visual observed and elaborately analyzed. Meanwhile, the effects of filling ratio, gravity action, and heating mode on the heat transfer performance were presented. This study indicates the effectiveness of micro-OHP as a promising candidate to act as ultra-compact heat exchanges applied to micro-cooling areas.
2. Description of the experiment 2.1. Fabrication and charging of the micro-OHP In this work, the micro-OHP, composed of a pair of h1 0 0i silicon wafer and a Pyrex 7740 glass plate, was fabricated by standard MEMS technology. Details of the whole fabrication procedure have been presented in our previous works [22,23]. As shown in Fig. 1 (a), the overall size of the micro-OHP is 30 mm 32 mm 1.025 mm and it has eleven U-turns at the evaporator section. The trapezoidal cross-section view of parallel microchannels and geometrical parameters were illustrated in Fig. 1(b). The top width, bottom width and depth of each microchannel are 650, 294, and 275 lm, respectively, related to a hydraulic diameter of about 357 lm. After being fabricated, the micro-OHP was partially filled with a dielectric liquid HFE-7100 (product from 3 M Corporation [31]) and its important thermophysical properties at the saturated state are listed in Table 1. Before the experiment, the micro-OHP was evacuated to a pressure of about 0.1 Pa inside, and then a proper amount of degassed HFE-7100 was backfilled into the micro device
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Fig. 1. (a) Image of the silicon-based micro-OHP and (b) schematic diagram of partial cross section of parallel microchannels with geometrical parameters.
Table1 Saturated thermophysical properties of HFE-7100 at 1 atm [31]. Fluid
Ts (°C)
ql (kg/m3)
kl (W/mK)
cp (J/kgK)
gl 106 (kg/ms)
r 103 (N/m)
hfg (kJ/kg)
HFE-7100
61
1510
0.075
1183
380
13.6
112
after evacuation. Since the micro-OHP performance is sensitive to the fluid filling ratio, accurate charging is quite important to package the micro-OHP. However, charging and packaging comes to be a tremendous challenge for this micro device because the total liquid volume in it is negligible, typically in the order of magnitude of about 10–100 lL, as compared with conventional large versions. To obtain a desired filling ratio, the charging process was implemented by a special flexible charging system in our lab as shown
in Fig. 2. Following steps are utilized for the evacuation and filling of this micro-OHP. (1) Open Valves 1, 3 and 5 and close Valves 2, 4 and 6. Fill the syringe with working fluid and push the fluid and its front non-condensable gases forward until the liquid-gas interface move to a position between Valves 1 and 3, and then close Valve 1. (2) Open Valves 2, 3, 5 and 6 and then evacuate the micro-OHP and the entire path using the vacuum pump while Valves 1 and 4 remain closed. Once the desired pressure (about 0.1 Pa) inside
Fig. 2. A photograph of experimental setup for micro-OHP degassing, filling and charging.
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the system is achieved, close Valve 2 and isolate the pump from the charging device and the micro-OHP. Then, close Valve 6 and turn off vacuum pump in turn. (3) Close all the valves and open Valves 1, 3 and 5 in turn. With the syringe connecting to the micro-OHP, the working fluid is pushed towards the micro-OHP by atmospheric pressure and fills it. (4) Close all the valves except for Valve 4 and the medical vacuum pump begins to work. Open Valve 5 until the fluid in the path between Valve 5 and the medical vacuum pump has been fully removed. Then, partially pump out a proper amount of working fluid using the medical vacuum pump and obtain the desired volume of working fluid inside the micro-OHP. Once the required fluid volume is achieved, close Valves 4, 5 and turn off the medical vacuum pump in turn. (5) Fuse the filling glass capillary tube connecting to the micro-OHP and package the device. In this way, the micro-OHP can be well sealed and the pressure-resistant capability satisfies the working condition. It is noted that this micro-OHP can be charged repeatedly if only the fused filling tube is replaced by a new one. To determine the filling ratio accurately, an image of the micro device was captured and the total liquid volume inside it can be estimated with an accuracy of about ±2% using the software of Pixel Ruler. Different filling ratios by volume ranging from about 31% to 72% were utilized for the present experimental study. 2.2. Experimental setup Fig. 3(a) presents the experimental setup, mainly consisting of a test section (MEMS-based micro-OHP), a multi-channel data acquisition system, a high-speed video system, a heating power supply unit and a liquid cooling unit. The micro-OHP has three parts, namely the evaporation, adiabatic and condensation sections with lengths of 6, 16 and 10 mm, respectively. In this study, the microOHP was placed at two orientations (0° and 90°) to investigate the gravity effect on its performance. As shown in Fig. 3(a), the evaporator was heated at the bottom by a film heater which was connected to a DC power supply (GPS-2303C, GW). The input voltage and current to the film heater were measured by two digital multimeters (VC890D, VICTOR), respectively, and then the heating power input could be calculated by the product of them. The condenser was cooled by ethylene glycol aqueous solution, which was pumped continuously from a cold bath (DC-0510, Shanghai Hengping Instrument and Meter). Besides, a flow meter was used to measure the flow rate of the cooling liquid. To measure the temperature distribution at the backside of silicon substrate, several T-type thermocouples (±0.1 °C accuracy, OMEGA) were used as illustrated in Fig. 3(b). T1, T2, T3 and T4 represent the evaporator temperatures, T5 and T6 represent the adiabatic temperatures, and T7 and T8 are the temperatures of condenser section. In addition, another two same T-type thermocouples were applied to measure the inlet/outlet temperatures of cooling fluid at the condenser. The output signals from these thermocouples were collected by a computerized data acquisition system (34972, Agilent). The heat removed from the condenser could be calculated by
Q out ¼ mcp ðT out T in Þ
ð1Þ
where m, cp, Tout and Tin are the mass flow rate, specific heat, and inlet and outlet temperatures of cooling liquid, respectively. Then, a thermal balance analysis was performed between the heat applied to the evaporator and the heat took away from the condenser. To reduce the heat loss from the test section to ambient, a fiberglass insulation of 8 mm thickness was fixed by a polycarbonate plate underneath the backside of the evaporation and adiabatic sections of the micro device. The comparison demonstrated that the total heat loss from the test section to the ambient was less than about 5.3%.
Fig. 3. Schematic of (a) the experiment system and (b) location of thermocouples.
To observe and record the oscillating motion and flow patterns in the micro-OHP, a high-speed video (including a CCD camera with a speed up to 20000 fps and a macro lens) was placed in the direct front of the test section. During the experiment test, the power input was stepwise increased until a quasi-steady state was established. Then, the spatial temperatures and power inputs could be recorded. 2.3. Data reduction and uncertainty analysis The thermal performance, an important parameter of the micro-OHP, is indicated by the overall thermal resistance, which is calculated by
R ¼ ðT e T c Þ=Q
ð2Þ
where Q (i.e. Q = UI) is the heating power input to the evaporator region, Te and Tc are the average temperatures of the evaporator and condenser, respectively. Table 2 lists the maximum uncertainties of main parameters in this study. The uncertainties of direct measurement parameters, such as Ti, U, and I, were synthesized by the system uncertainty
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Te
Tc
U
I
Q
R
Maximum uncertainties (%)
0.21
0.35
0.85
0.81
6.24
6.50
es from the precision of instruments and the random uncertainty eR from the repeatability of data as follows:
e¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi e2s þ e2R
ð3Þ
The uncertainties of the indirect measurement parameters such as Q and R were obtained in light of the error propagation principle. According to the detailed uncertainty analysis reported in Ref. [32], the maximum uncertainty of thermal resistance in this study was about 6.5%. 3. Results and discussion 3.1. Thermal performance of the MEMS-based micro-OHP Fig. 4 shows the thermal resistance of micro-OHP at different filling ratios both at the vertical and horizontal orientations. Compared to conventional tube OHPs, the thermal resistances are relatively higher and mainly attributable to the low ratio of the area occupied by channels to silicon cross sectional area, and it is only 21.2% in the present study. As a result, a considerable amount of heat was transferred directly by conduction through the substrate and lead to large thermal resistances. At relatively low power inputs, the influence of filling ratio on the thermal performance seems to be irregular, whereas at higher power inputs, an optimal filling ratio of about 40% with respect to the lowest thermal resistance was observed at the vertical orientation associated with a maximum heat flux of about 20.3 W/cm2, indicating better thermal performance. When the applied power input is greater than a threshold value as shown in the enlarged part of Fig. 4(a), the thermal resistance decreased sharply due to the sudden vigorous fluid movement in the micro-OHP, implying the start-up [8]. It is noted that the increase of filling ratio from 31% to 53% alter the start-up power input slightly and there is no obvious increase until the filling ratio was further increased to 65% and 72% as given in Table 3. At relatively higher filling ratios, the fluid motion in the micro-OHP tended to be restrained by the increased flow resistance and the reduced movement space. As a result, there was only local or even no fluid movement in the micro-OHP, and the thermal performance deteriorated markedly. Therefore, it can be concluded that the optimal filling ratio is between 40% and 53% at the vertical orientation if both of the start-up power input and thermal performance were evaluated together. In addition, the thermal resistance increased as increasing the power input from about 8.5 W to 12.7 W, and then decreased as power inputs were greater than 12.7 W at the filling ratio of 31% (see Fig. 4(a)), which is largely attributed to the hysteresis effect [33] stemmed from the partial dry-out and liquid slug stagnancy as observed in the evaporator region. The hysteresis effect mainly depends on the slug/plug distribution and the vapor expansion is sometimes incapable of driving the slug/plug movement in the device till the power input reached above 12.7 W. Then, the fluid motion stagnancy was terminated and oscillation reappeared in channels of the micro device, contributing to high performance. In Fig. 4, it is obvious that the thermal performance of microOHP at the vertical orientation is much better than that at the horizontal orientation, associated with lower thermal resistances, indicating the influence of gravity could not be ignored. As for
Fig. 4. Thermal resistance versus heating power input of the micro-OHP at different filling ratios: (a) vertical orientation and (b) horizontal orientation.
Table 3 Start-up power inputs at different filling ratios of the micro-OHP. Filling ratio
31%
40%
53%
65%
72%
Start-up power input (W)
5.2
5.3
5.5
6.8
8.2
the two-phase fluid flow and heat transfer in microchannels, the gravity effect can be partially or totally neglected [34]. According to the classification of microchannel proposed by Cheng et al. qffiffiffiffiffiffiffiffiffiffiffiffiffiffi [35], the Bond number Bo ¼ D ðql rqv Þg should satisfy an inequation of Bo2 < 0:5, therefore the value of Bo less than 0.224 is required to ignore the gravity effect. However, a smaller Bo will also remarkably magnify the difficulty of micro-OHP startup and even lead to startup failure eventually. In fact, this micro-OHP failed to start-up and subsequently no oscillation was observed inside it when it was horizontally orientated at the lowest filling ratio of 31% (see Fig. 4(b)). The early occurrence of dry-out at this filling ratio account for it as the liquid in the condenser could not return to the evaporator timely without the aid of gravity. Similar phenomenon also appeared at higher filling ratios of 65% and 72% because of the significant increase in flow resistance as well as the movement space limitation of working
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fluid inside the micro-OHP. At the horizontal orientation, however, the micro-OHP can still work featured by robust self-sustained oscillation inside it when the filling ratios are 40% and 53%. As shown in Fig. 4(b), the start-up power inputs are about 9.5 W and 11.1 W at the filling ratio of 40% and 53%, respectively, much higher than at the vertical orientation (see Fig. 4(a)), indicating the active role that gravity plays to boost the micro-OHP start-up just like in large versions. According to the visual observation, the oscillation amplitude and frequency of fluid movement increased with increasing the power input and thus the thermal resistance reduced remarkably. However, the dry-out occurred under the power input greater than 21.5 W at the filling ratio of 40%, resulting in a rapid increase of thermal resistance ultimately. The experiment was terminated when the evaporator temperature exceeded 150 °C and hence the dry-out was not observed at the filling ratio of 53%. This result provides an indicative maximum heat transfer capacity of the micro-OHP at the filling ratio of 40%
when it was horizontally placed, while a larger heat transport limitation appeared at the 53% filling ratio. 3.2. Hydrodynamic behavior of the MEMS-based micro-OHP 3.2.1. Slug/plug (bubble) oscillating motion After the start-up of micro-OHP, the transition toward the quasi-steady oscillation state would be completed quickly as a further increase of power input. Fig. 5 depicts the liquid-slug/vaporplug (bubble) distribution in the micro-OHP at a transient state after start-up. Since obvious and sustained circulation flow was not observed in the entire micro-OHP, the evolution of flow
Fig. 5. High-speed snapshot of a full view of micro-OHP during the quasi-steady oscillation state.
Fig. 6. Displacement variations of the liquid/vapor interface marked in the left image during the alternate ‘‘SAOP-LAOP” behavior versus time under a power input of 15.8 W at the filling ratio of 65%.
Fig. 7. Time lapse for alternate ‘‘SAOP-LAOP” behavior of slug/plugs in the vertical oriented micro-OHP: (a) FR = 40%, Q = 15.9 W; (b) FR = 53%, Q = 16.0 W; (c) FR = 65%, Q = 15.8 W.
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characteristics and slug/plug distribution within each microchannel almost kept in step with adjacent microchannels characterized by a phase lead or lag. Therefore, the oscillating motion in the whole array of parallel microchannels could be approximately represented by a single bend (denoted as ‘‘A” in Fig. 5) within a period of time. According to the consecutive images, the displacement variation of the liquid/vapor interface versus time in the ‘‘A” region was obtained using the software Proanalyst according to a similar method of our most recent work [20]. In Fig. 6, the Y-axis, namely the value of ‘L’, represents the relative position of the liquid/vapor interface from the bottom of U-turn in the micro-OHP. It is found that the stable oscillation of slug/plug within the ‘‘A” region could be approximately divided into two stages of interest, namely the small-amplitude oscillation phase (SAOP, indicated by ‘‘0”) and the large-amplitude oscillation phase (LAOP, indicated by ‘‘1”). The former is characterized by slug/plug oscillation with a value of L smaller than La + (Le + Lc)/2, while the latter represents the robust oscillation of slugs/plugs with a value of L larger than La + (Le + Lc)/2 and the liquid/vapor interface then could even move across the bend and enter into adjacent channels, leading to an ephemeral circulation flow in several U-turns as evidenced in Fig. 6. Noted that the blue and cyan dashed lines in LAOP region served as the symbol of ephemeral oscillation flow observed in channels. Moreover, the ephemeral circulation flow was often disrupted by local flow direction switch, produced by bubble nucleation triggering quick vapor expansion at the evaporator, resulting in flow reversals in circulating flow regimes. The random and intermittent occurrence of local dryout at relatively high power inputs may partially account for this classification. Actually, the surface tension induced rewetting process would mostly replenish the strong film evaporation at the hot region and thus avoid the thorough dry-out. Fig. 7 displays the alternate ‘‘SAOP-LAOP” behavior of working fluid under the power input of about 16.0 W at three different filling ratios. It can be seen that the time ratio of LAOP in the whole period decreases with increasing the filling ratio, which in turn increases the corresponding ratio of SAOP. Herein, the average period of ‘‘SAOP-LAOP‘‘ alternation can be defined as
Dt Dt ¼ N
Fig. 8. Image sequences of flow behavior in the vertically oriented micro-OHP at different filling ratios: (a) FR = 40%, Q = 15.9 W; (b) FR = 53%, Q = 16.0 W; and (c) FR = 65%, Q = 15.8 W (V, A/S, I, S, B and L simply represent the vapor phase, annular/ semi-annular flow, injection flow, slug flow, bubbly flow and liquid phase, respectively).
ð4Þ
where N is the total number of LAOP in a given time of Dt. Within the time of 4.5 s in Fig. 7, the values of N are 8, 6 and 6 associated with Dt equal to 0.55 s, 0.75 s and 0.75 s at the filling ratios of 40%, 53% and 65%, respectively. It implies that the heat transfer capacity of the micro device can be enhanced as the filling ratio decreases from 65% to 40% due to the prolonged LAOP of the working fluid. In addition, the ephemeral circulation was more easily observed at medium filling ratios (40% and 53%) coupled with larger heating power inputs. 3.2.2. Flow behavior in the micro-OHP Fig. 8 shows the transient internal flow behavior in the vertically-placed micro-OHP at different filling ratios within the
Fig. 9. Sketch of flow patterns at different locations inside the micro-OHP (from ‘a’ to ‘d’ represent the annular/semi-annular flow, injection flow, slug flow, and bubbly flow, respectively).
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average periods of ‘‘SAOP-LAOP‘‘ alternation presented in Fig. 7. Different flow patterns, including the bubbly flow, slug flow, injection flow and annular/semi-annular flow, were observed in this micro device. The alternate ‘‘SAOP-LAOP” behavior began with a rapid expansion of vapor plug as a power input of about 16.0 W was applied to the evaporator and immediately gave rise to the annular/semi-annular flow. As shown in Fig. 8(a), the annular/ semi-annular flow could even move across the bend and enter into an adjacent microchannel and the ephemeral circulation flow was observed in several channels of the micro-OHP. Besides, the plug flow derived from the expansion of small bubbles or split from elongated-bubble, and the injection flow as reported in Ref. [24] appeared in the adiabatic section. At the condenser section, it was normally occupied by dispersed tiny or Taylor bubbles. Owning to the increase of time ratio related to LAOP, more heat could be transferred from the evaporator to the condenser, and therefore reducing the temperature difference between these two sections. However, the LAOP of working fluid in the micro-OHP would turn into SAOP and subsequently increase the temperature difference. If the temperature difference was elevated to a threshold value, the driving potential originated from the pressure difference between the hot and cold ends would become large enough to ignite the self-sustained oscillation again and then began the next ‘‘SAOPLAOP” alternation. Generally, the flow pattern has a significant impact on the micro-OHP performance. Compared to the slug and injection flow, the annular/semi-annular flow is linked to higher heat transfer coefficient [36,37]. At the filling ratio of 40% (see Fig. 8(a)), more channels at the evaporator and adiabatic sections were occupied by annular/semi-annular flow as compared to at filling ratios of 53% (Fig. 8(b)) and 65% (Fig. 8(c)) and therefore contributed to higher thermal performance as evidenced in Fig. 4. To understand the variation of flow pattern distribution, Fig. 9 presents a sketch of typical flow patterns at different regions inside a channel of the micro-OHP. Aforementioned flow patterns were illustratively provided. At the evaporator section, the inner channel wall was covered with a thin evaporating film surrounding the vapor core and the nucleation of bubbles in the thin film was intermittently observed at relatively high power inputs. The liquid film would undergo temporal dry-out due to complete evaporation when the liquid rewetting process was not sufficiently refreshed to maintain a balance against the vigorous film evaporation and resultant local dry patches appeared randomly. However, the oscillating motion can drive out the dry-out patches rapidly and prevent the continued rise in temperature until stable surface burnout occur due to a vapor blanket on the surface. At the condenser section, the bubble or dispersed bubble flow was observed due to the vapor plug shrinkage or collapse. The boundaries of different flow patterns, quantitatively depicted by coordinate locations along the channel length as illustrated in Fig. 8, were approximately determined according to Eq. (5), written as
L¼
10 X 22 X Lij =220
channels at the same time. With the code written in Matlab 7.0, four contour maps associated with different flow patterns at different power inputs along the longitudinal locations of micro-OHP from the evaporator to the condenser are presented in Fig. 10.
ð5Þ
j¼1 i¼1
where Lij is the coordinate location along the channel length of i-th microchannel (i = 1, 2, 3, . . ., 22 represents the channel number from left to right of the micro-OHP as shown in Fig. 1(a)) at the time of (t + 0.2j) s. Noted that t signifies the initial time referred to oscillation in the alternate ‘‘SAOP-LAOP” behavior as shown in Fig. 7, and the value of j equals to 1, 2, 3, . . ., 10. Actually, Lij is calculated herein with the aid of software Pixel Ruler. According to the visual video sequences, the boundaries of different flow patterns could be estimated using Eq. (5). Besides, the boundaries of different flow patterns during the ephemeral circulation flow in several channels of the micro-OHP were replaced by the average values in other
Fig. 10. Contour maps of different flow patterns varying heating power input along the longitudinal locations from the evaporator to the condenser: (a) FR = 31%, (b) FR = 40%, (c) FR = 53%, and (d) FR = 65% (V, A/S, I, S, B and L simply represent the vapor phase, annular/semi-annular flow, injection flow, slug flow, bubbly flow and liquid phase, respectively).
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Fig. 10 gives the approximate overall distribution of aforementioned flow patterns in all microchannels at different filling ratios. At relatively low power inputs (less than about 12 W), a non-uniform and significant change in the flow pattern distribution is observed, which is consistent with the thermal resistance as described in Fig. 4. According to the visual observation, however, it was confirmed that the evaporator was usually occupied by annular/semi-annular flow, and the adiabatic section and condenser were governed by slug and bubbly flows, respectively. It is noted that the zone of annular/semi-annular, injection and slug flows was expanded at higher power inputs until increased up to a threshold value, after which the power input only slightly affects the flow pattern distribution. The corresponding threshold values of power inputs are about 12.7, 16.5, 12.4 and 15.2 W at the filling ratios of 31%, 40%, 53% and 65%, respectively. The smaller the filling ratio is, there exist a larger zone of annular/semi-annular flow. Especially, the average length of annular/semi-annular flow accounts for the half and one third of the channel length at the filling ratio of 40% and 65%, respectively. Therefore, the whole region of annular/semi-annular flow in the channels from the evaporator to condenser displays a negative correlation with the thermal resistance of the micro-OHP. In the present experiment, the complete dry-out only occurred at relatively lower filling ratios and higher power inputs, largely focused on the lowest filling ratio of 31% as shown in Fig. 10(a). In addition, Fig. 10(d) evidences a partial dry-out occurred in the evaporator at relatively larger filling ratios and lower power inputs due to the start-up failure, corresponding to the increase of thermal resistance as shown in Fig. 4(a).
4. Conclusions In this study, an experimental investigation was performed to understand the operational characteristics of an MEMS-based micro-OHP partially charged with HFE-7100. Trapezoidal channels with a hydraulic diameter of 357 lm were wet-etched on a silicon wafer to form the meandering closed-loop micro-OHP with eleven turns. The main conclusions are drawn as follows: (1) At the vertical orientation, the micro-OHP could startup and work robustly at a filling ratio ranging from 30% to 75% and there existed an optimal filling ratio with respect to both the thermal performance and start-up power input, which is between 40% and 53%. However, at the horizontal orientation, this micro-OHP can function as a micro pulsating device and effectively transfer the heat only at a filling ratio range limited from 40% to 55%. (2) The alternate ‘‘SAOP-LAOP” behavior was observed and highlighted to understand the thermo-hydrodynamic variation of the micro-OHP, and the performance enhancement was largely attributable to the prolonged LAOP due to the reduction of filling ratio from 65% to 40%. (3) Annular/semi-annular flow, injection/slug flow, and bubbly flow were observed largely at the evaporator section, adiabatic section and condenser section, respectively. The ephemeral circulation flow and nucleation of bubbles occurred intermittently. The whole region of annular/semi-annular flow from the evaporator to the condenser displays a negative correlation with the thermal resistance of the micro-OHP. The occurrence of dryout satisfied the condition of relative lower filling ratios and higher power inputs, largely focused on the lowest filling ratio of 31%.
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Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 51206065).
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Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Operational characteristics of an MEMS-based micro oscillating heat pipe Qin Sun a, Jian Qu b,⇑, Jianping Yuan a, Qian Wang b a b
Research Center of Fluid Machinery Engineering and Technology, Jiangsu University, Zhenjiang 212013, China School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China
h i g h l i g h t s The thermo-fluidic characteristics of an MEMS-based micro-OHP were experimentally investigated. The micro-OHP worked well at a Bond number less than 0.4 and the gravity action still functioned. Alternation of small- and large-amplitude oscillation phase was observed in the micro-OHP. 2
The micro-OHP could sustain a maximum heat flux up to about 20.3 W/cm .
a r t i c l e
i n f o
Article history: Received 13 February 2017 Revised 18 May 2017 Accepted 21 June 2017 Available online 22 June 2017 Keywords: Micro oscillation heat pipe Thermal performance Start-up Gravity effect Dry-out
a b s t r a c t The flow characteristics and thermal performance of an MEMS-based micro oscillating heat pipe (microOHP) were experimentally investigated using dielectric liquid HFE-7100 as working fluid. The micro-OHP was integrated on a silicon wafer with trapezoidal channels having a hydraulic diameter of 357 lm. The gravity action on the micro-OHP performance functioned greatly and could not be ignored. High-speed visual observation demonstrated the annular/semi-annular flow, injection/slug flow, and bubbly flow in the micro-OHP. The alternate ‘‘small-amplitude oscillation phase (SAOP)—large-amplitude oscillation phase (LAOP)” was verified in this micro-OHP at the quasi-steady oscillation state, and its performance improvement was attributable to the extended time ratio of LAOP. In addition, the ephemeral circulation flow and bubble nucleation were observed intermittently. The occurrence of dry-out mainly appeared at lower filling ratios and higher power inputs. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction The goal of enhancing heat transfer while reducing the size and volume of energy conversion/thermal management systems has been the subject of intensive research for several decades. However, challenges such as growing energy demands, the need for increased energy efficiency and material savings, increased functionality and ease of unit handling as well as space limitations for device packaging have led to advanced research into the development of compact heat exchangers, mainly for the thermal management of high performance electronic devices. To provide advanced thermal control solution for nextgeneration electronic devices, persistent efforts have been made to develop novel micro cooling technologies capable of removing large amount of heat from chips or improving temperature
⇑ Corresponding author. E-mail address: [email protected] (J. Qu). http://dx.doi.org/10.1016/j.applthermaleng.2017.06.109 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
uniformity [1–4]. Among them, oscillating heat pipe (OHP), a high-efficiency two phase heat transfer device invented by Akachi [5], is considered as a promising solution and has been attracted considerable attention recent years [6–13] due to the simple structure, high performance, and acceptable reliability. Traditionally, OHPs are made from serpentine-arranged, interconnected capillary tubes. Although the OHP structure is quite simple, the thermo-fluidic operational characteristic is highly non-linear and extremely complex at the capillary level [14,15]. In an OHP, the bubble growth and expansion at the hot region and simultaneous bubble shrinking and collapse at the cold region provide a pumping action to transport entrapped liquid slugs in a complex pulsating–translating–vibratory fashion [16], resulting in self-sustained thermally driven flow oscillations. In addition to the latent heat, considerable amount of sensible heat transfer also occurs in the OHP, i.e., Taylor bubble train flow at pulsating condition and the resultant slug-plug distribution contribute significantly to the net heat transport [17]. To reveal the operational
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Nomenclature Bo cp D es eR g hfg I L m N Q R t T U
Bond number specific heat (J/(kgK)) hydraulic diameter (m) system uncertainty random uncertainty gravitational acceleration (m/s2) vaporization latent heat (kJ/kg) input current (A) length (m) mass flow rate (kg/s) number of LAOP behavior heating power (W) thermal resistance (°C/K) time (s) temperature (°C) input voltage (V)
mechanism of OHPs, some studies on flow visualization mainly using transparent glass OHPs have been conducted. Tong et al. [6] performed the flow visualization for a closed loop glass OHP filled with 60% methanol. It was observed that the working fluid oscillates with large amplitude during the start-up process, however, at the quasi-steady state, the working fluid circulated and coupled with complex process such as nucleation boiling and coalescence of bubbles. Bulk circulation flow and local flow direction switch in a multi-turn OHP were also observed and verified by Xu et al. [9]. In addition, they found that both of the bubble displacement and velocity displayed quasi-sine oscillating waves in the methanol-charged OHP, while the water-charged OHP had quasi-rectangular shape for bubble displacements. Senjaya and Inoue [18] investigated the boiling phenomena in the evaporator section of an OHP, and found that the sudden and acceleration motion of liquid-vapor interfaces is mainly triggered by the tubesized bubble generation and growth, which also provides the large driving force to support the random slug/plug motions. To further understand the internal thermo-hydrodynamic phenomena in OHPs, some other efforts have been made to explore the thermofluidic characteristics of OHPs [17,19,20]. Due to the wickless style, OHPs are suitable for miniaturization and could be fabricated and integrated directly into semiconductor chips on the basis of micro-electromechanical system (MEMS) technology, acting as thermal spreaders. In addition, our recent theoretical analysis [21] demonstrated that the low limit of internal diameter for an OHP could be less than 0.3 mm for its operation when proper working fluids are selected. In recent years, tenacious efforts, mainly experimentally, have also been made to understand the internal thermo-hydrodynamic behavior of micro-OHPs. Qu et al. [22–24] tentatively designed and fabricated several microOHPs on silicon substrates with trapezoidal channels, characterized by hydraulic diameters ranging from 251 to 393 lm. Experimental tests demonstrated that these micro devices could operate normally with sustained oscillatory flow and function satisfactorily. At an allowable temperature of 109 °C, an overall rather than local heat flux up to 10.7 W/cm2 was achieved for the FC-72 charged micro-OHP with each channel having a hydraulic diameter of 352 lm [23]. To understand the complex oscillating flow in micro-OHPs, Yoon and Kim [25] conducted a flow visualization experiment and proposed a vapor spring-liquid mass model to explain the oscillating mechanisms. Inspired by non-uniform channel configuration OHP design applicable to all orientations proposed by Wang and his co-workers [26,27], non-uniform or
Greek
r q
k
g
surface tension (N/m) density (kg/m3) thermal conductivity (W/(mK)) dynamic viscosity (kg/(ms))
Subscript a adiabatic section c condenser section e evaporator section in inlet l liquid out outlet s saturation v vapor 1, 2, . . .,8 thermocouple number
alternate channel has recently been developed by Yang et al. [28,29] to facilitate the start-up and improve the micro-OHP performance. Kwon and Kim [30] also conducted an experimental study to investigate the effect of dual-diameter (or alternate) channel on the flow and heat transfer characteristics of silicon-based micro-OHPs. Experimental results demonstrated that the microOHPs with dual-diameter channels have better thermal performance and can operate independently irrespective of the orientation as compared to the micro-OHP with uniform channels. However, the underlying thermo-fluidic behavior is quite complex at the micro-scale level and yet to be fully revealed, requiring further experimental investigations to shed light on the fluid flow and heat transfer characteristics to provide a deep understanding of it. In this paper, the flow behavior, slug/plug oscillation characteristics, and flow pattern evolution subject to different heat input levels in an MEMS-based micro-OHP fabricated on silicon wafer were visual observed and elaborately analyzed. Meanwhile, the effects of filling ratio, gravity action, and heating mode on the heat transfer performance were presented. This study indicates the effectiveness of micro-OHP as a promising candidate to act as ultra-compact heat exchanges applied to micro-cooling areas.
2. Description of the experiment 2.1. Fabrication and charging of the micro-OHP In this work, the micro-OHP, composed of a pair of h1 0 0i silicon wafer and a Pyrex 7740 glass plate, was fabricated by standard MEMS technology. Details of the whole fabrication procedure have been presented in our previous works [22,23]. As shown in Fig. 1 (a), the overall size of the micro-OHP is 30 mm 32 mm 1.025 mm and it has eleven U-turns at the evaporator section. The trapezoidal cross-section view of parallel microchannels and geometrical parameters were illustrated in Fig. 1(b). The top width, bottom width and depth of each microchannel are 650, 294, and 275 lm, respectively, related to a hydraulic diameter of about 357 lm. After being fabricated, the micro-OHP was partially filled with a dielectric liquid HFE-7100 (product from 3 M Corporation [31]) and its important thermophysical properties at the saturated state are listed in Table 1. Before the experiment, the micro-OHP was evacuated to a pressure of about 0.1 Pa inside, and then a proper amount of degassed HFE-7100 was backfilled into the micro device
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Fig. 1. (a) Image of the silicon-based micro-OHP and (b) schematic diagram of partial cross section of parallel microchannels with geometrical parameters.
Table1 Saturated thermophysical properties of HFE-7100 at 1 atm [31]. Fluid
Ts (°C)
ql (kg/m3)
kl (W/mK)
cp (J/kgK)
gl 106 (kg/ms)
r 103 (N/m)
hfg (kJ/kg)
HFE-7100
61
1510
0.075
1183
380
13.6
112
after evacuation. Since the micro-OHP performance is sensitive to the fluid filling ratio, accurate charging is quite important to package the micro-OHP. However, charging and packaging comes to be a tremendous challenge for this micro device because the total liquid volume in it is negligible, typically in the order of magnitude of about 10–100 lL, as compared with conventional large versions. To obtain a desired filling ratio, the charging process was implemented by a special flexible charging system in our lab as shown
in Fig. 2. Following steps are utilized for the evacuation and filling of this micro-OHP. (1) Open Valves 1, 3 and 5 and close Valves 2, 4 and 6. Fill the syringe with working fluid and push the fluid and its front non-condensable gases forward until the liquid-gas interface move to a position between Valves 1 and 3, and then close Valve 1. (2) Open Valves 2, 3, 5 and 6 and then evacuate the micro-OHP and the entire path using the vacuum pump while Valves 1 and 4 remain closed. Once the desired pressure (about 0.1 Pa) inside
Fig. 2. A photograph of experimental setup for micro-OHP degassing, filling and charging.
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the system is achieved, close Valve 2 and isolate the pump from the charging device and the micro-OHP. Then, close Valve 6 and turn off vacuum pump in turn. (3) Close all the valves and open Valves 1, 3 and 5 in turn. With the syringe connecting to the micro-OHP, the working fluid is pushed towards the micro-OHP by atmospheric pressure and fills it. (4) Close all the valves except for Valve 4 and the medical vacuum pump begins to work. Open Valve 5 until the fluid in the path between Valve 5 and the medical vacuum pump has been fully removed. Then, partially pump out a proper amount of working fluid using the medical vacuum pump and obtain the desired volume of working fluid inside the micro-OHP. Once the required fluid volume is achieved, close Valves 4, 5 and turn off the medical vacuum pump in turn. (5) Fuse the filling glass capillary tube connecting to the micro-OHP and package the device. In this way, the micro-OHP can be well sealed and the pressure-resistant capability satisfies the working condition. It is noted that this micro-OHP can be charged repeatedly if only the fused filling tube is replaced by a new one. To determine the filling ratio accurately, an image of the micro device was captured and the total liquid volume inside it can be estimated with an accuracy of about ±2% using the software of Pixel Ruler. Different filling ratios by volume ranging from about 31% to 72% were utilized for the present experimental study. 2.2. Experimental setup Fig. 3(a) presents the experimental setup, mainly consisting of a test section (MEMS-based micro-OHP), a multi-channel data acquisition system, a high-speed video system, a heating power supply unit and a liquid cooling unit. The micro-OHP has three parts, namely the evaporation, adiabatic and condensation sections with lengths of 6, 16 and 10 mm, respectively. In this study, the microOHP was placed at two orientations (0° and 90°) to investigate the gravity effect on its performance. As shown in Fig. 3(a), the evaporator was heated at the bottom by a film heater which was connected to a DC power supply (GPS-2303C, GW). The input voltage and current to the film heater were measured by two digital multimeters (VC890D, VICTOR), respectively, and then the heating power input could be calculated by the product of them. The condenser was cooled by ethylene glycol aqueous solution, which was pumped continuously from a cold bath (DC-0510, Shanghai Hengping Instrument and Meter). Besides, a flow meter was used to measure the flow rate of the cooling liquid. To measure the temperature distribution at the backside of silicon substrate, several T-type thermocouples (±0.1 °C accuracy, OMEGA) were used as illustrated in Fig. 3(b). T1, T2, T3 and T4 represent the evaporator temperatures, T5 and T6 represent the adiabatic temperatures, and T7 and T8 are the temperatures of condenser section. In addition, another two same T-type thermocouples were applied to measure the inlet/outlet temperatures of cooling fluid at the condenser. The output signals from these thermocouples were collected by a computerized data acquisition system (34972, Agilent). The heat removed from the condenser could be calculated by
Q out ¼ mcp ðT out T in Þ
ð1Þ
where m, cp, Tout and Tin are the mass flow rate, specific heat, and inlet and outlet temperatures of cooling liquid, respectively. Then, a thermal balance analysis was performed between the heat applied to the evaporator and the heat took away from the condenser. To reduce the heat loss from the test section to ambient, a fiberglass insulation of 8 mm thickness was fixed by a polycarbonate plate underneath the backside of the evaporation and adiabatic sections of the micro device. The comparison demonstrated that the total heat loss from the test section to the ambient was less than about 5.3%.
Fig. 3. Schematic of (a) the experiment system and (b) location of thermocouples.
To observe and record the oscillating motion and flow patterns in the micro-OHP, a high-speed video (including a CCD camera with a speed up to 20000 fps and a macro lens) was placed in the direct front of the test section. During the experiment test, the power input was stepwise increased until a quasi-steady state was established. Then, the spatial temperatures and power inputs could be recorded. 2.3. Data reduction and uncertainty analysis The thermal performance, an important parameter of the micro-OHP, is indicated by the overall thermal resistance, which is calculated by
R ¼ ðT e T c Þ=Q
ð2Þ
where Q (i.e. Q = UI) is the heating power input to the evaporator region, Te and Tc are the average temperatures of the evaporator and condenser, respectively. Table 2 lists the maximum uncertainties of main parameters in this study. The uncertainties of direct measurement parameters, such as Ti, U, and I, were synthesized by the system uncertainty
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Te
Tc
U
I
Q
R
Maximum uncertainties (%)
0.21
0.35
0.85
0.81
6.24
6.50
es from the precision of instruments and the random uncertainty eR from the repeatability of data as follows:
e¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi e2s þ e2R
ð3Þ
The uncertainties of the indirect measurement parameters such as Q and R were obtained in light of the error propagation principle. According to the detailed uncertainty analysis reported in Ref. [32], the maximum uncertainty of thermal resistance in this study was about 6.5%. 3. Results and discussion 3.1. Thermal performance of the MEMS-based micro-OHP Fig. 4 shows the thermal resistance of micro-OHP at different filling ratios both at the vertical and horizontal orientations. Compared to conventional tube OHPs, the thermal resistances are relatively higher and mainly attributable to the low ratio of the area occupied by channels to silicon cross sectional area, and it is only 21.2% in the present study. As a result, a considerable amount of heat was transferred directly by conduction through the substrate and lead to large thermal resistances. At relatively low power inputs, the influence of filling ratio on the thermal performance seems to be irregular, whereas at higher power inputs, an optimal filling ratio of about 40% with respect to the lowest thermal resistance was observed at the vertical orientation associated with a maximum heat flux of about 20.3 W/cm2, indicating better thermal performance. When the applied power input is greater than a threshold value as shown in the enlarged part of Fig. 4(a), the thermal resistance decreased sharply due to the sudden vigorous fluid movement in the micro-OHP, implying the start-up [8]. It is noted that the increase of filling ratio from 31% to 53% alter the start-up power input slightly and there is no obvious increase until the filling ratio was further increased to 65% and 72% as given in Table 3. At relatively higher filling ratios, the fluid motion in the micro-OHP tended to be restrained by the increased flow resistance and the reduced movement space. As a result, there was only local or even no fluid movement in the micro-OHP, and the thermal performance deteriorated markedly. Therefore, it can be concluded that the optimal filling ratio is between 40% and 53% at the vertical orientation if both of the start-up power input and thermal performance were evaluated together. In addition, the thermal resistance increased as increasing the power input from about 8.5 W to 12.7 W, and then decreased as power inputs were greater than 12.7 W at the filling ratio of 31% (see Fig. 4(a)), which is largely attributed to the hysteresis effect [33] stemmed from the partial dry-out and liquid slug stagnancy as observed in the evaporator region. The hysteresis effect mainly depends on the slug/plug distribution and the vapor expansion is sometimes incapable of driving the slug/plug movement in the device till the power input reached above 12.7 W. Then, the fluid motion stagnancy was terminated and oscillation reappeared in channels of the micro device, contributing to high performance. In Fig. 4, it is obvious that the thermal performance of microOHP at the vertical orientation is much better than that at the horizontal orientation, associated with lower thermal resistances, indicating the influence of gravity could not be ignored. As for
Fig. 4. Thermal resistance versus heating power input of the micro-OHP at different filling ratios: (a) vertical orientation and (b) horizontal orientation.
Table 3 Start-up power inputs at different filling ratios of the micro-OHP. Filling ratio
31%
40%
53%
65%
72%
Start-up power input (W)
5.2
5.3
5.5
6.8
8.2
the two-phase fluid flow and heat transfer in microchannels, the gravity effect can be partially or totally neglected [34]. According to the classification of microchannel proposed by Cheng et al. qffiffiffiffiffiffiffiffiffiffiffiffiffiffi [35], the Bond number Bo ¼ D ðql rqv Þg should satisfy an inequation of Bo2 < 0:5, therefore the value of Bo less than 0.224 is required to ignore the gravity effect. However, a smaller Bo will also remarkably magnify the difficulty of micro-OHP startup and even lead to startup failure eventually. In fact, this micro-OHP failed to start-up and subsequently no oscillation was observed inside it when it was horizontally orientated at the lowest filling ratio of 31% (see Fig. 4(b)). The early occurrence of dry-out at this filling ratio account for it as the liquid in the condenser could not return to the evaporator timely without the aid of gravity. Similar phenomenon also appeared at higher filling ratios of 65% and 72% because of the significant increase in flow resistance as well as the movement space limitation of working
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fluid inside the micro-OHP. At the horizontal orientation, however, the micro-OHP can still work featured by robust self-sustained oscillation inside it when the filling ratios are 40% and 53%. As shown in Fig. 4(b), the start-up power inputs are about 9.5 W and 11.1 W at the filling ratio of 40% and 53%, respectively, much higher than at the vertical orientation (see Fig. 4(a)), indicating the active role that gravity plays to boost the micro-OHP start-up just like in large versions. According to the visual observation, the oscillation amplitude and frequency of fluid movement increased with increasing the power input and thus the thermal resistance reduced remarkably. However, the dry-out occurred under the power input greater than 21.5 W at the filling ratio of 40%, resulting in a rapid increase of thermal resistance ultimately. The experiment was terminated when the evaporator temperature exceeded 150 °C and hence the dry-out was not observed at the filling ratio of 53%. This result provides an indicative maximum heat transfer capacity of the micro-OHP at the filling ratio of 40%
when it was horizontally placed, while a larger heat transport limitation appeared at the 53% filling ratio. 3.2. Hydrodynamic behavior of the MEMS-based micro-OHP 3.2.1. Slug/plug (bubble) oscillating motion After the start-up of micro-OHP, the transition toward the quasi-steady oscillation state would be completed quickly as a further increase of power input. Fig. 5 depicts the liquid-slug/vaporplug (bubble) distribution in the micro-OHP at a transient state after start-up. Since obvious and sustained circulation flow was not observed in the entire micro-OHP, the evolution of flow
Fig. 5. High-speed snapshot of a full view of micro-OHP during the quasi-steady oscillation state.
Fig. 6. Displacement variations of the liquid/vapor interface marked in the left image during the alternate ‘‘SAOP-LAOP” behavior versus time under a power input of 15.8 W at the filling ratio of 65%.
Fig. 7. Time lapse for alternate ‘‘SAOP-LAOP” behavior of slug/plugs in the vertical oriented micro-OHP: (a) FR = 40%, Q = 15.9 W; (b) FR = 53%, Q = 16.0 W; (c) FR = 65%, Q = 15.8 W.
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characteristics and slug/plug distribution within each microchannel almost kept in step with adjacent microchannels characterized by a phase lead or lag. Therefore, the oscillating motion in the whole array of parallel microchannels could be approximately represented by a single bend (denoted as ‘‘A” in Fig. 5) within a period of time. According to the consecutive images, the displacement variation of the liquid/vapor interface versus time in the ‘‘A” region was obtained using the software Proanalyst according to a similar method of our most recent work [20]. In Fig. 6, the Y-axis, namely the value of ‘L’, represents the relative position of the liquid/vapor interface from the bottom of U-turn in the micro-OHP. It is found that the stable oscillation of slug/plug within the ‘‘A” region could be approximately divided into two stages of interest, namely the small-amplitude oscillation phase (SAOP, indicated by ‘‘0”) and the large-amplitude oscillation phase (LAOP, indicated by ‘‘1”). The former is characterized by slug/plug oscillation with a value of L smaller than La + (Le + Lc)/2, while the latter represents the robust oscillation of slugs/plugs with a value of L larger than La + (Le + Lc)/2 and the liquid/vapor interface then could even move across the bend and enter into adjacent channels, leading to an ephemeral circulation flow in several U-turns as evidenced in Fig. 6. Noted that the blue and cyan dashed lines in LAOP region served as the symbol of ephemeral oscillation flow observed in channels. Moreover, the ephemeral circulation flow was often disrupted by local flow direction switch, produced by bubble nucleation triggering quick vapor expansion at the evaporator, resulting in flow reversals in circulating flow regimes. The random and intermittent occurrence of local dryout at relatively high power inputs may partially account for this classification. Actually, the surface tension induced rewetting process would mostly replenish the strong film evaporation at the hot region and thus avoid the thorough dry-out. Fig. 7 displays the alternate ‘‘SAOP-LAOP” behavior of working fluid under the power input of about 16.0 W at three different filling ratios. It can be seen that the time ratio of LAOP in the whole period decreases with increasing the filling ratio, which in turn increases the corresponding ratio of SAOP. Herein, the average period of ‘‘SAOP-LAOP‘‘ alternation can be defined as
Dt Dt ¼ N
Fig. 8. Image sequences of flow behavior in the vertically oriented micro-OHP at different filling ratios: (a) FR = 40%, Q = 15.9 W; (b) FR = 53%, Q = 16.0 W; and (c) FR = 65%, Q = 15.8 W (V, A/S, I, S, B and L simply represent the vapor phase, annular/ semi-annular flow, injection flow, slug flow, bubbly flow and liquid phase, respectively).
ð4Þ
where N is the total number of LAOP in a given time of Dt. Within the time of 4.5 s in Fig. 7, the values of N are 8, 6 and 6 associated with Dt equal to 0.55 s, 0.75 s and 0.75 s at the filling ratios of 40%, 53% and 65%, respectively. It implies that the heat transfer capacity of the micro device can be enhanced as the filling ratio decreases from 65% to 40% due to the prolonged LAOP of the working fluid. In addition, the ephemeral circulation was more easily observed at medium filling ratios (40% and 53%) coupled with larger heating power inputs. 3.2.2. Flow behavior in the micro-OHP Fig. 8 shows the transient internal flow behavior in the vertically-placed micro-OHP at different filling ratios within the
Fig. 9. Sketch of flow patterns at different locations inside the micro-OHP (from ‘a’ to ‘d’ represent the annular/semi-annular flow, injection flow, slug flow, and bubbly flow, respectively).
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average periods of ‘‘SAOP-LAOP‘‘ alternation presented in Fig. 7. Different flow patterns, including the bubbly flow, slug flow, injection flow and annular/semi-annular flow, were observed in this micro device. The alternate ‘‘SAOP-LAOP” behavior began with a rapid expansion of vapor plug as a power input of about 16.0 W was applied to the evaporator and immediately gave rise to the annular/semi-annular flow. As shown in Fig. 8(a), the annular/ semi-annular flow could even move across the bend and enter into an adjacent microchannel and the ephemeral circulation flow was observed in several channels of the micro-OHP. Besides, the plug flow derived from the expansion of small bubbles or split from elongated-bubble, and the injection flow as reported in Ref. [24] appeared in the adiabatic section. At the condenser section, it was normally occupied by dispersed tiny or Taylor bubbles. Owning to the increase of time ratio related to LAOP, more heat could be transferred from the evaporator to the condenser, and therefore reducing the temperature difference between these two sections. However, the LAOP of working fluid in the micro-OHP would turn into SAOP and subsequently increase the temperature difference. If the temperature difference was elevated to a threshold value, the driving potential originated from the pressure difference between the hot and cold ends would become large enough to ignite the self-sustained oscillation again and then began the next ‘‘SAOPLAOP” alternation. Generally, the flow pattern has a significant impact on the micro-OHP performance. Compared to the slug and injection flow, the annular/semi-annular flow is linked to higher heat transfer coefficient [36,37]. At the filling ratio of 40% (see Fig. 8(a)), more channels at the evaporator and adiabatic sections were occupied by annular/semi-annular flow as compared to at filling ratios of 53% (Fig. 8(b)) and 65% (Fig. 8(c)) and therefore contributed to higher thermal performance as evidenced in Fig. 4. To understand the variation of flow pattern distribution, Fig. 9 presents a sketch of typical flow patterns at different regions inside a channel of the micro-OHP. Aforementioned flow patterns were illustratively provided. At the evaporator section, the inner channel wall was covered with a thin evaporating film surrounding the vapor core and the nucleation of bubbles in the thin film was intermittently observed at relatively high power inputs. The liquid film would undergo temporal dry-out due to complete evaporation when the liquid rewetting process was not sufficiently refreshed to maintain a balance against the vigorous film evaporation and resultant local dry patches appeared randomly. However, the oscillating motion can drive out the dry-out patches rapidly and prevent the continued rise in temperature until stable surface burnout occur due to a vapor blanket on the surface. At the condenser section, the bubble or dispersed bubble flow was observed due to the vapor plug shrinkage or collapse. The boundaries of different flow patterns, quantitatively depicted by coordinate locations along the channel length as illustrated in Fig. 8, were approximately determined according to Eq. (5), written as
L¼
10 X 22 X Lij =220
channels at the same time. With the code written in Matlab 7.0, four contour maps associated with different flow patterns at different power inputs along the longitudinal locations of micro-OHP from the evaporator to the condenser are presented in Fig. 10.
ð5Þ
j¼1 i¼1
where Lij is the coordinate location along the channel length of i-th microchannel (i = 1, 2, 3, . . ., 22 represents the channel number from left to right of the micro-OHP as shown in Fig. 1(a)) at the time of (t + 0.2j) s. Noted that t signifies the initial time referred to oscillation in the alternate ‘‘SAOP-LAOP” behavior as shown in Fig. 7, and the value of j equals to 1, 2, 3, . . ., 10. Actually, Lij is calculated herein with the aid of software Pixel Ruler. According to the visual video sequences, the boundaries of different flow patterns could be estimated using Eq. (5). Besides, the boundaries of different flow patterns during the ephemeral circulation flow in several channels of the micro-OHP were replaced by the average values in other
Fig. 10. Contour maps of different flow patterns varying heating power input along the longitudinal locations from the evaporator to the condenser: (a) FR = 31%, (b) FR = 40%, (c) FR = 53%, and (d) FR = 65% (V, A/S, I, S, B and L simply represent the vapor phase, annular/semi-annular flow, injection flow, slug flow, bubbly flow and liquid phase, respectively).
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Fig. 10 gives the approximate overall distribution of aforementioned flow patterns in all microchannels at different filling ratios. At relatively low power inputs (less than about 12 W), a non-uniform and significant change in the flow pattern distribution is observed, which is consistent with the thermal resistance as described in Fig. 4. According to the visual observation, however, it was confirmed that the evaporator was usually occupied by annular/semi-annular flow, and the adiabatic section and condenser were governed by slug and bubbly flows, respectively. It is noted that the zone of annular/semi-annular, injection and slug flows was expanded at higher power inputs until increased up to a threshold value, after which the power input only slightly affects the flow pattern distribution. The corresponding threshold values of power inputs are about 12.7, 16.5, 12.4 and 15.2 W at the filling ratios of 31%, 40%, 53% and 65%, respectively. The smaller the filling ratio is, there exist a larger zone of annular/semi-annular flow. Especially, the average length of annular/semi-annular flow accounts for the half and one third of the channel length at the filling ratio of 40% and 65%, respectively. Therefore, the whole region of annular/semi-annular flow in the channels from the evaporator to condenser displays a negative correlation with the thermal resistance of the micro-OHP. In the present experiment, the complete dry-out only occurred at relatively lower filling ratios and higher power inputs, largely focused on the lowest filling ratio of 31% as shown in Fig. 10(a). In addition, Fig. 10(d) evidences a partial dry-out occurred in the evaporator at relatively larger filling ratios and lower power inputs due to the start-up failure, corresponding to the increase of thermal resistance as shown in Fig. 4(a).
4. Conclusions In this study, an experimental investigation was performed to understand the operational characteristics of an MEMS-based micro-OHP partially charged with HFE-7100. Trapezoidal channels with a hydraulic diameter of 357 lm were wet-etched on a silicon wafer to form the meandering closed-loop micro-OHP with eleven turns. The main conclusions are drawn as follows: (1) At the vertical orientation, the micro-OHP could startup and work robustly at a filling ratio ranging from 30% to 75% and there existed an optimal filling ratio with respect to both the thermal performance and start-up power input, which is between 40% and 53%. However, at the horizontal orientation, this micro-OHP can function as a micro pulsating device and effectively transfer the heat only at a filling ratio range limited from 40% to 55%. (2) The alternate ‘‘SAOP-LAOP” behavior was observed and highlighted to understand the thermo-hydrodynamic variation of the micro-OHP, and the performance enhancement was largely attributable to the prolonged LAOP due to the reduction of filling ratio from 65% to 40%. (3) Annular/semi-annular flow, injection/slug flow, and bubbly flow were observed largely at the evaporator section, adiabatic section and condenser section, respectively. The ephemeral circulation flow and nucleation of bubbles occurred intermittently. The whole region of annular/semi-annular flow from the evaporator to the condenser displays a negative correlation with the thermal resistance of the micro-OHP. The occurrence of dryout satisfied the condition of relative lower filling ratios and higher power inputs, largely focused on the lowest filling ratio of 31%.
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Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 51206065).
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