Proc. IV Conferência Nacional em Mecânica dos Fluidos, Termodinâmica e Energia, LNEC, Lisboa, Portugal, 2012
Flow boiling instabilities in transparent microchannels Vânia Silvério1, António L.N. Moreira2 Universidade Técnica de Lisboa, Instituto Superior Técnico, Departamento de Engenharia Mecânica, Av. Rovisco Pais 1049-001 Lisboa, Portugal 1,2 Center for Innovation, Technology and Policy Research, IN+ email:
[email protected] http://in3.dem.ist.utl.pt/microchannel Summary The methanol and ethanol phase change from liquid to vapor is studied in horizontal circular transparent glass single channels with 542m internal diameter, externally coated with a transparent metallic film that allows at the same time the optical access to the phenomena and the application of a constant heat flux. The study of the behavior of bubbles formed within the channel in diabatic conditions and instabilities caused by these are characterized using averaged temperature measurements along the outer wall of the channel, pressure and temperature at the inlet and outlet plenums and of high-speed images acquired in situ. Keywords: microchannel, phase change, high-speed visualization, two-phase flow instabilities.
1 INTRODUCTION Boiling in microchannels is considered an asset in the removal of heat to the microscale level, as it provides an efficient way of fluid movement, due to the smaller necessity of pumping power required when compared with single-phase liquid flow to achieve a certain amount of heat removal. Also, the higher ability of heat removal when compared with single-phase liquid flow is of major importance. It has diverse applications at a microscale level whether in computer electronics, aerospace components or automotive electronic parts, due also to their compactness. Nevertheless, two-phase flow boiling is still poorly understood by researchers, particularly with respect to the flow instabilities and their influence on heat transfer. The processes of heat transmission and hydrodynamics are different from those recorded for the macroscale [Peng and Wang, 1993, Kew and Cornwell, 2002], and as such, only some of the knowledge can be applied directly. Physical phenomena such as thin film evaporation and bubble confinement are subjects with high relevance in microscale flows. At this scale, the flow is thought to be dominated by forces of surface tension, capillary effects and wall effects; a bubble that forms can occupy the entire section of the canal, which opposes to what happens in major dimension channels, where a large number of bubbles can coexist in the flow. An important aspect of flow boiling in microchannels is the fluctuation of pressure, because it can lead to instabilities. Two-phase flow instabilities become undesirable as they promote temperature oscillations with high amplitudes, premature critical heat flux and mechanical vibrations [Bogojevic et al., 2009], channel dry-out and reverse flow [Brutin et al., 2003], all with highly adverse effect on heat transfer in the microchannel cooling system, hence the need for these phenomena to be well understood and predicted. Previous studies on two-phase flow instabilities in single circular channels, both experimental and theoretical aimed the fundamental understanding of boiling phenomena and safety of boiling heat exchangers. Most of the studies found in literature have been carried out in multiple parallel channels or single channel, but with non uniform heat flux, as one of the channel walls is usually non-heated for visualization purposes. Balasubramanian and Kandlikar (2005) studied the water flow in a stack of six parallel rectangular microchannels with individual hydraulic diameter of 333m and associated the pressure fluctuations to different flow characteristics and their effects on flow instability. Wang et al (2008) investigated the effects of three different inlet/outlet configurations on water flow boiling instabilities in parallel microchannels with Dh=186m and found that amplitudes of pressure and temperature are dependent on these configurations. Single rectangular microchannel flow has been addressed by Galvis and Culham (2012) that found flow patterns to be dependent on the mass flux, heat flux and channel size and the amplitude of pressure oscillation increased with the heat flux. Yen et al. (2006) investigated boiling in single transparent glass square and circular microchannels observing semi-periodic variation in the flow patterns due to the confined space that limited the bubble growth in the radial
1
direction of the channel. Barber et al. (2009) induced boiling in a vertical single transparent glass rectangular microchannel under constant heat flux conditions. They found that correlation of pressure oscillations with temperature fluctuations as a function of the heat flux applied to the microchannel was possible. They noted various phenomena during the flow instabilities and characterized them into different timescales. This work focuses on flow instabilities, using measurements of pressure drop, temperature profiles and image acquisition with a high speed camera. With the experimental setup used in the study is possible to obtain transmission coefficients of local heat and associate it with events captured in the flow visualization.
2 EXPERIMENTAL METHODOLOGY The scheme of experimental apparatus is shown in Figure 1. It consists of a borosilicate glass circular section channel with Dh=542m (Vitrocom®, uncertainty, u ±10%), 100mm long and wall thickness 100m, electrically heated by means of a DC power supply (GEN150-5, from TDK-Lambda®, u < ±0.14%) connected to the transparent film of indium oxide deposited on the channel outer wall, by means of a conductive glue. The heated length starts at L=20mm and has 60mm long. TC air1 TC 1
V, I
TC 2
TC 3
TC 4
TC 5
flow
TC 6
TC 7
TC 8
TC 9
TC 10
TC 11
TC 12
TC air2 TC 14 TC 13
test section 20mm
60mm
20mm
Pout
Pin valve thermostatic bath
Lhyd TC TB
Lsat
valve
microscope + high-speed camera TC R
filter syringe pump
reservoir balance
Fig. 1 Scheme of the experimental setup Pressure and Temperature are both measured at the channel inlet and outlet (stagnation chambers). Temperature measurements along the channel outer wall are obtained with 25m k-type thermocouples (u < ±2%), and air temperature measurements are obtained in the vicinity of the inlet and outlet of the channel. Two pressure transducers ECO1 from Wika® (0-4bar, u < ± 0.5%) are used for pressure measurements. Temperature and pressure data are collected and recorded at a rate of 4096 points per second during 30s using PCI-6024E/BNC2120 and NI-USB6008 data acquisition systems from National Instruments with Labview interface and Matlab ® code for processing. The subcooled fluid flow (Tf,in < Tf,sat) is controlled by the New Era® 1010 syringe pump (u = ±1%). Images of the inner flow are acquired at a rate of 8196 frames per second during 3.16s with a Vision Research Phantom® V4.2 high-speed camera coupled to an inverted DMIL LED Leica® microscope with Hi-Plan 10x (scale factor 0.22) and C-mount 0.55x HC lenses. Since the pressure transducers were connected to the inlet and outlet stagnation chambers, the measured pressure drop is the sum of pressure drops across the inlet stagnation chamber, microchannel, outlet stagnation chamber, as well as the contraction and expansion. These last two are found to be very small when compared to the total pressure drop, and therefore discard. Also, the pressure transducer at the inlet measures the pressure of the liquid phase (the fluid enters the channel at Tf,in = 297K), whereas at the outlet is measuring the two-phase boiling, where both liquid and vapor are present. Hence the two-phase contribution should be added through either a homogeneous approach or a Lockhart-Martinelli multiplication factor [Kandlikar, 2006]. However, the results shown here illustrate the exact values obtained by the pressure sensors at the inlet and outlet of the microchannel during two-phase flow boiling in kPa
p poutlet pinlet
(1)
The heat transfer coefficient in the saturation region of the flow, h, is calculated according to:
2
h
q s" Ts ,in,avg T f ,avg
(2)
where Ts,in,avg is the average temperature at the inner surface of the channel; Tf,avg is the average temperature of the fluid and q”s is the heat flux transferred to the fluid calculated from the applied electrical power, q”app, and accounting for heat losses by natural convection, q”conv,air and by radiation, q”rad,air from the outer surface of the "
InOx thin film to the environment ( q s
" " " qapp qconv , air q rad, air ) as found in Silvério and Moreira (2008).
These heat losses are determined assuming that in the absence of inner flow, the channel reaches thermal equilibrium when the heat is completely dissipated by convection and radiation to the surrounding air. Both the averaged pressure drop, pavg and the average temperatures Ts,in,avg and Tf,avg are found by numerical integration applying the Simpson Rule. The quality, , can be solved by using equation (3) where LHT is the heated length [m], G is the mass flux [kg.m-2.s-1], Dh is the hydraulic diameter [m], hl is the enthalpy of the liquid @ Tinlet [J.kg-1], hsl is the enthalpy of the saturated liquid [J.kg-1] and hfg is the latent heat of vaporization [J.kg-1].
q" L
1
4 s HT hl hsl G Dh h fg
(3)
3 RESULTS In order to study the influence of surface average temperature on flow instability, the experiments are conducted by increasing the heat flux with a constant mass flux, for six mass fluxes, three different heat fluxes and two different fluids, methanol, CH3OH (99.8%, BDH®) and ethanol C2H5OH (absolute, Emparta®). Subcooled liquid is supplied to the channel at 297K, reaches fully develop hydrodynamic conditions at Lhyd and enters the heated section 20mm downstream. The saturation length Lsat is defined as the distance within the heated section where the fluid reaches the saturation temperature. Lsat is calculated from the energy balance using the heat flux transferred to the fluid and values are displayed in Table 1 for methanol entering the channel at inlet mass flux of 227kg.m-2.s-1 and in Table 2 for ethanol at inlet mass flux of 228kg.m-2.s-1. It is worth noting that Lsat is negligibly small compared with the overall heated length (60mm). The high-speed images are acquired in the saturation region only. Table 1. Saturation Length for CH3OH at inlet mass flux G = 227kg.m-2.s-1 Average surface Heat Flux Hydrodynamic Length Saturation Length temperature q”s [W.cm-2] Lhyd [mm] Lsat [mm] Ts,avg [K] 5.12 346.8 6.06 6.74 366.31 5.84 4.61 9.29 380.09 3.34
Table 2. Saturation Length for C2H5OH at inlet mass flux G = 228kg.m-2.s-1 Average surface Heat Flux Hydrodynamic Length Saturation Length temperature ” -2 q s [W.cm ] Lhyd [mm] Lsat [mm] Ts,avg [K] 6.39 359.05 6.37 9.38 365.48 2.89 4.34 13.34 372.70 3.05 Temporal variations of pressure drop of methanol (1) and ethanol (2) are shown in Figure 2 for increasing mass flux (a-b-c) at constant heat flux and in Figure 3 for increasing heat flux (a-b-c) at constant mass flux.
3
20
20
15
15
15
10 5 0 0
10
2
4 6 Time [s]
8
0 0
10
5 2
1b)
4 6 Time [s]
8
0 0
10
2
4 6 Time [s]
8
10
2
4 6 Time [s]
8
10
1c)
s 20
20
15
15
15
10
10
5
5
0 0
0 0
2
4 6 Time [s]
8
P [kPa]
20 P [kPa]
P [kPa]
C2H5OH
q
10 5
1a)
”
P [kPa]
20 P [kPa]
P [kPa]
CH3OH
The quasi-periodic behavior of pressure drop shown in Figures 2 and 3 has also been detected in the temperatures measured along the channel wall and is associated with semi-periodic variations of the flow pattern as observed in high-speed images.
10
5 2
4 6 Time [s]
8
0 0
10
2c)
2b)
2a)
10
12
12
10
10
10
8 6 0
4 6 Time [s]
8
8 6 0
10
2
1b)
4 6 Time [s]
8
8 6 0
10
12
10
10
10
6 0
2a)
2
4 6 Time [s]
8
10
P [kPa]
12
8
8 6 0
2b)
2
4 6 Time [s]
2
4 6 Time [s]
8
10
2
4 6 Time [s]
8
10
1c)
12 P [kPa]
P [kPa]
C2H5OH
G
2
1a)
P [kPa]
12 P [kPa]
P [kPa]
CH3OH
Fig. 2 Plots of pressure drop, p, in kPa over 10s during two-phase flow boiling of methanol for G=132kg.m-2.s-1 and heat flux (1a) 4.5W.cm-2, (1b) 6.6W.cm-2 and (1c) 9.2W.cm-2 and ethanol for G=228kg.m-2.s-1 and heat flux (2a) 6.0W.cm-2, (2b) 8.8W.cm-2 and (2c) 12.4W.cm-2.
8
10
8 6 0
2c)
Fig. 3 Plots of pressure drop, p, in kPa, over 10s during two-phase flow boiling of 1) Methanol, q”s=9.2W.cm-2 and 2) Ethanol, q”s=8.8W.cm-2. a) 132kg.m-2.s-1, b) 227kg.m-2.s-1 and c) 321kg.m-2.s-1 A qualitative picture can be given as follows: bubbles start nucleating right before Lsat and quickly grow into slugs due to the very limited space in the microchannel for bubbles to grow freely; depending on the fluid, magnitude of the heat flux and mass flux, these bubbles may grow in the downstream direction only or in the downstream and upstream direction simultaneously; pressure variations induced by phase change interact with the flow and result in a cyclic pressure-flow feedback during which the flow pattern sucessively evolves from a liquid flow pattern, into a bubbly flow, slug flow, annular flow, dryout and liquid refill. For high values of the heat flux, chaotic, unstable non-periodic flow instabilities are produced. Hetsroni et al. (2003) reported a similar quasi-periodic behavior in a microchannel heat sink, and related it with the significant volume change of bubbles, that rappidly fill the entire channel cross section. For both fluids it is found that the specific heat flux (heat flux per mass of fluid, q”s/G) increases the frequency of the oscillations, which may be associated with an increase of the bubble nucleation frequency as reported by Balasubramanian and Kandlikar (2005). Worth noting is the fact that, simultaneously, the amplitude of the oscillations decreases. In microchannels, the combined effect of the heat flux and the mass flux is characterized by the non-dimensional Boiling number, which is defined as Bo = q”s/(G.hfg). Figure 4 displays the average pressure drop as a function of the boiling number, Bo, for methanol (left) and ethanol (right) and three values of the wall heat flux. In
4
general the results show that, for small values of Bo, the pressure drop decreases steeply down to a characteristic minimum associated with incipient boiling in the stable flow regime, more pronounced in the flow of methanol; a further increase of Bo from the minimum increases the pressure drop due to vapor generation. The results in Figure 4 also show that the average pressure drop increases with increasing heat flux for both methanol and ethanol as expected. 18
CH3OH
C2H5OH
16
4.5W.cm-2
16
6.0W.cm-2
14
6.6W.cm-2
14
8.8W.cm-2
12
9.2W.cm-2
12
12.4W.cm-2
10 8 6
4
Pressure Drop [kPa]
Pressure Drop [kPa]
18
10 8 6 4
2
2
0
0 0
0.0005 Bo
0
0.001
0.0005 Bo
0.001
Fig. 4 Average pressure drop during two-phase flow boiling of methanol (left) and ethanol (right) for 3 different heat fluxes (q”s) and Tinlet = 297K. Fast Fourier Transform (FFT) analysis was performed on pressure drop data to obtain the frequency distribution (Fig. 5a). This analysis shows that the frequency of oscillations increases with increasing boiling number indicating an increase in the cyclic behavior (Fig 5b). Increasing the heat flux (or decreasing the mass flux), the cycle - formation, growth and dryout of bubbles - becomes more rapid and more frequent. As the dryout occurs, the channel is clear for new liquid refill, giving rise to a new cycle. For methanol, higher dominant frequencies are found. The properties of both fluids are thought to be on the base of such differences. Dominant frequency [Hz]
12
1 Amplitude
0.8 0.6
f = 6.16Hz
0.4 0.2 0 0
CH3OH, 9.2W.cm-2 C2H5OH, 8.8W.cm-2
10 8 6 4 2 0
5 10 Frequency [Hz]
15
0
0.0005 Bo
0.001
Fig. 5 a) Pressure drop frequency analysis for CH3OH, G=321kg.m-2.s-1, q”s = 9.4W.cm-2 and b) Dependence of dominant pressure frequency on boiling number The peak amplitude of pressure drop oscillations, (Pmax-Pmin)/2, is also dependent on the boiling number (Figure 6). The stable flow regime (Bo < 12.6x10-05 or q”s/G < 0.17kJ.kg-1 for methanol and Bo < 17.3x10-05 or q”s/G < 0.20kJ.kg-1 for ethanol) characterized by small oscillations of pressure drop, includes single-phase flow and incipient boiling when small isolated bubbles flow inside microchannels in a pattern which can be described as the bubbly flow regime. Additional increase in Bo results in high oscillations, characteristic of slow transitions between small slugs and elongated plugs that fill the entire channel. The posterior decrease of magnitude of flow oscillations for higher Bo is attributed to the lower residence time of elongated bubbles inside the microchannel. The transition between flow regimes becomes faster, hence diminuishing the the magnitude of oscillations.
5
CH3OH 4.5W.cm-2
5
6.6W.cm-2
9.2W.cm-2
4 3 2
1
Magnitude of p oscillations [kPa]
Magnitude of p oscillations [kPa]
6
6
C2H5OH
6.0W.cm-2
5
8.8W.cm-2
12.4W.cm-2
4 3 2
1 0
0 0
0.0005 Bo
0
0.001
0.0005 Bo
0.001
Fig. 6 Magnitude of pressure drop oscillations (Pmax-Pmin)/2 during two-phase flow boiling of methanol (left) and ethanol (right) for 3 different surface heat fluxes (q”s) and Tinlet = 297K. The dependence of the flow boiling average heat transfer coefficient, h, on the heat to mass flux ratio may be evaluated by means of the boiling number (Figure 7a) or the vapor quality. h vs the local vapor quality near the channel heated outlet is presented in Figure 7b. The maximum values found for ethanol are in accordance to that found by Diaz and Schmidt (2007). For the previous authors, the heat transfer coefficient was found to be influenced by convective boiling, as the heat transfer coefficient increases in the region of subcooled boiling and decreases immediately after the quality reaches zero. C2H5OH 6.0W.cm-2 C2H5OH 8.8W.cm-2
[W.m-2.K-1]
10,000
h
12,000
4,000
8,000 6,000
14,000
C2H5OH 6.0W.cm-2 C2H5OH 8.8W.cm-2
12,000
h [W.m-2.K-1]
14,000
10,000 8,000 6,000 4,000 2,000
2,000 0
0
0.0005 Bo
0.001
0 -0.2
-0.1
0 0.1 Quality [-]
0.2
0.3
Fig. 7 Boiling heat transfer coefficient versus a) boiling number and b) local quality near the channel heated outlet for different q”s It is important to note that the increase in the heat transfer is coincident with the lower pressure drops, but higher magnitude of pressure oscillations. The equilibrium between the applied heat and mass flux is preponderant in the efficiency of the heat removal and the consequent oscillations generated in both pressure and temperature measurements. Depending on operating conditions, two types of behavior can be observed, a steady state characterized by fluctuations in pressure drop with low amplitude and a multiphase flow in non-stationary characteristic frequency without or with higher amplitude. In the present study two types of flow instabilities were identified, those originating low frequency and high amplitude and those originating high frequency and low amplitude in both the measured temperature and pressure data. High-speed imaging showed that the fluctuations existed due to cyclic behavior between liquid/two-phase/vapor flow. Bogojevic et al. (2009) reported similar trends for a stack of 40 parallel rectangular microchannels using water as the working fluid and constant heat flux conditions. They found that these oscillations were dependent on the heat to mass flux ratio and inlet fluid temperature.
6
Pressure [kPa]
130
Pinlet Poutlet
125 120 115 0
0.2
0.4 0.6 Time [s]
0.8
1
P [kPa]
20 15 10 5 0 0
0.2
0.4 0.6 Time [s]
0.8
1
Fig. 8 (a) Close up of Pin and Pout pressure fluctuations during flow boiling instability of methanol, taken over 1000ms (b) high-speed imaging taken over 1 cycle of 2.7ms. Experimental conditions: Limage= 37.48mm @ q”s=6.4W.cm-2 and G =510kg.m-2.s-1. Figure 8 is an example of the phenomena at the threshold of instability. The average pressure drop is 7.8kPa and q”s/G 0.12. The peak amplitude is found to be 0.4kPa. The images are taken from bottom of the channel and the bulk flow is from left to rigth. The fenomenon is relatively quick. Each cycle takes on average 2.7ms. The pressure at the inlet decreases, and a vapor bubble is formed near the wall at a distance 37-38mm from the channel inlet, changing the geometry of the flow field. This bubble grows freely until it is confined by the channel diameter. When it is found confined by the channel diameter, proceeds to block the channel cross section causing an increase in the pressure measured at the channel inlet. After this point, the fast growth of the bubble is noticed over time. It is evident by the images that the bubble nose growth is faster than the bubble displacement to the point where the bubble nose starts to recede. At this point, the liquid-vapor interface at the bubble tail, previously with a concave shape, turns to flat due mainly to the resistance from the fluid inlet flow. The interface then breaks down and small bubbles can be seen further in the channel. Care must be taken when visualizing the flow at the channel outlet, as it may lead to erroneous conclusions due to the bubbles resemblance to bubbly flow. When increasing the heat flux or decreasing the mass flux, the nucleated bubble begins to grow until the “onset of vapor blockage” is observed. At this point the pressure increases both upstream and downstream. In the previous case, the bubble breaks down, leading pressure to diminuish. In this case, the elongated bubble increases in size, mainly due to the higher heat flux/lower mass flux, and pressure increase at both inlet and outlet is observed. To explain how the bubble dynamics relate to the pressure peaks, a close up of both inlet and outlet pressures over a time of 1000ms is presented in Figures 9 and 10. It is also annotated to allow the relationship between the pressure fluctuations and the bubble dynamics in the microchannel, namely bubble nucleation, bubble growth and vapor slug flow. For Figure 9, the average pressure drop is 10.2kPa and q”s/G 0.14. The peak amplitude is found to be 3.6kPa. Each cycle takes on average 570ms. The lower values of Pinlet and Poutlet correspond to liquid flow or bubbly flow (1-2). The bubble is formed near the wall at a distance of 20.47mm from the channel heated inlet. This bubble grows freely until it is confined by the channel diameter as previously reported. The rise in pressure is due to vapor blockage (3). The growth of the vapor bubble to the downstream direction has higher influence on Poutlet than on Pinlet (4). The flat bubble tail displacement follows the fluid displacement as the growing bubble nose pushes the fluid in the downstream direction leading to accentuated pressure fluctuations on Poutlet (5). The bubble is then purged from the channel and pressure values decrease to their original baseline
7
0.57s
Pressure [kPa]
130 125
(4)
120 (1)
Pinlet
(1)
Poutlet
(2)
(5)
bubble growth onset of vapour blockage
(3)
(6)
(2)
bubbly flow
elongated bubble growth
(4) bubble growth and displacement
(5)
115 0
(3)
0.5 Time [s]
1
(6)
bubbly flow
Fig. 9 (a) Close up of Pin and Pout pressure fluctuations during flow boiling of methanol, taken over 1000ms (b) ” -2 high-speed imaging taken from 1 cycle of 570ms. Experimental conditions: Limage= 20.09mm @ q s=6.97W.cm -2 -1 and G =510kg.m .s . When the vapor production in the channel is high enough, an overpressure is induced leading to vapor recoil. Figure 10 shows temporal variations of the inlet and outlet pressures at a heat flux of 6.8W.cm-2 and an inlet mass flux of methanol of 510kg.m-2.s-1. The average pressure drop found is 9.01kPa and the relation q”s/G 0.13. The peak amplitude is found to be 3.6kPa. The reported fenomenon is also relatively slow. Each cycle takes on average 680ms. 0.68s
Pressure [kPa]
130
Pinlet Poutlet
125 (4)
120
bubble nucleation
onset of vapour blockage
(2) (3)
elongated bubble growth
(5)
(3)
(1)
115 0
(1)
(4) (2)
0.5 Time [s]
1
(5)
bubble expansion channel starts to refill with liquid
Fig. 10 Close up of Pin and Pout pressure fluctuations during flow boiling of methanol, taken over 1000ms annotated with corresponding high-speed imaging. Experimental conditions: Limage= 12.57mm @ q”s=6.8W.cm-2 and G =510kg.m-2.s-1. The vapor bubble nucleates at a distance of 15.19mm from the channel heated inlet. The bubble forms (1), grows and becomes confined (2) by the channel diameter as described earlier. The sharp increase in pressure, reported more clearly in Poutlet, is responsible for the sharp growth of the bubble (3), leading to vapor recoil, pushing the bubble to expand both upstream and downstream (4). The channel is then filled in with liquid and pressure returns to its baseline values and stabilizes. The cycle is reinitiated. To note that both in Figure 9 and 10 the pressure measurements at the channel outlet are more sensitive to the phenomena than the measured values for pressure at the inlet. However, in the case where an artificial nucleation site occurs within the flow, it is noted that pressure at inlet becomes much more sensitive to the phenomena. The artificial nucleation site (Figure 11) usually becomes active after vapor recoil and liquid refill and deactivates after 100ms. Figure 11 shows the temporal variations of the inlet and outlet pressures and average surface temperature at an heat flux of 5.89W.cm-2 and an inlet mass flux of methanol of 227kg.m-2.s-1. The average pressure drop found is 2.25kPa and the relation q”s/G 0.26. The peak amplitude is found to be 3.65kPa. Each cycle takes on average 250ms. Left images represent 2.2ms from the initial event and the images on the right represent the last 2.2ms of the event. The receding liquid-vapor contact line dries the artificial nucleation site, as the average temperature of the wall increases by 2K and the pressure drop increases by 3.8kPa. The liquid-vapor interface is then pushed forward, and the dry-spot is rewetted by the incoming liquid (left image #1). The velocity of the liquid-vapor interface is
8
that imposed by the syringe pump. At this point, nucleation starts, and becomes very vigorous after only 1.2ms (left image #10). The interface velocity increases due to the pressure exerted by the bubbles formation. The pressure is maintained in the upper baseline. The outlet pressure presents a smoother pressure variation than Pinlet due to the fact that the liquid-vapor interface is acting as a buffer. The temperature decreases sharply and is maintained in the lower baseline, evidencing the increase in heat removal due to phase change. After 97.8ms the artificial nucleation site is becoming less active (right image #1), and eventually ceases activity after 99.8ms (right image #16).
Pressure [kPa]
120
Pinlet
118
Poutlet
116 114 112 0
0.5 Time [s]
1
Ts,avg [K]
349
data1
348 347 346 345 0
0.5 Time [s]
1
Fig. 11 Close up of Pin and Pout pressure fluctuations during flow boiling of methanol and average temperature taken over 1000ms annotated with corresponding high-speed imaging. Experimental conditions: Limage= 51.2mm @ q”s=5.89W.cm-2 and G =227kg.m-2.s-1. Another type of instability found was thin film nucleate boiling, previously reported by Balasubramanian and Kandlikar (2005). Figure 12 shows bubbles nucleating in the thin liquid film surrounding an elongated vapor bubble and the destabilization of the elongated bubble. The phenomenon is very fast, as it takes about 122s for bubbles to form and grow, due to the high ratio q”s/G 0.8. The high heat flux and low mass flux promotes the passage from liquid to vapor. The bubble nucleates in the thin liquid film till it reaches the vapor slug, at which point it starts to coalesce. Because the growth is so sudden, it causes the slug liquid-vapor interface to move, causing a destabilization of the interface as seen in the images.
Fig. 12 High-speed images of thin film nucleate boiling. Experimental conditions: Limage= 55.01mm to 56.77mm ” -2 @ q s=10.5W.cm and G =132kg.m-2.s-1. Leica® Hi-plan 10x/0.22 microscope lens
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4 CONCLUSIONS The understanding of the phenomena of boiling in microchannels is crucial to many applications. Studies of boiling instabilities and specifically during the nucleation of bubbles within the microchannels are of particular interest, especially when accompanied by in-situ viewing of the phenomena. In this study, fluctuations in temperature and pressure measurements are associated with the flow regimes observed using a high speed camera. The results allow fluctuations relate to different flow characteristics, and their effect on instability. Different types of instabilities were reported for different ratio q”s/G. The bubble dynamics occurring in the microchannel is responsible for these instabilities, corroborated by pressure and temperature fluctuations. As the heat flux increases or the mass flux decreases, the time period of the inlet and outlet pressure fluctuations decreases. In other words, with an increase in heat flux or decrease in mass flux there is an increase in the fluctuation frequency of the pressure and temperature peaks. The lower values of averaged pressure drop at lower Bo correspond to those of both higher magnitude of pressure oscillations and higher heat transfer, however slight changes in Bo leads to dramatic changes in both pressure drop and heat transfer. Periods of instabilities or temperature excursions, even very small or short timescale, are important in microscale heat transfer. The fast growth leading to bubble confinement and collapse of the vapor bubbles in regions adjacent to the solid surfaces may cause surface damages, compromise the flow velocity and compromise the heat removal. In microchannel heat sinks composed of several microchannels, the liquid refill may be blocked by vapor, promoting the fast increase in surface temperature and eventually the collapse of the microchannel system. The knowledge of instability thresholds is therefore of utmost importance for the design and performance of microchannel heat sinks.
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