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An experimental study of flow boiling instability in a single microchannel
International Communications in Heat and Mass Transfer 35 (2008) 1229–1234
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
An experimental study of flow boiling instability in a single microchannel☆ Guodong Wang, Ping Cheng ⁎ Ministry of Education Key Laboratory of Power Machinery and Engineering, School of Mechanical and Power Engineering, Shanghai Jiaotong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Available online 9 September 2008 Keywords: Boiling instability Heat transfer Microchannels Instability Reversed flow Single microchannel
a b s t r a c t A simultaneous visualization and measurement study has been carried out to investigate stable and unstable flow boiling phenomena of deionized water in a single microchannel having a hydraulic diameter of 155 µm with a bottom Pyrex glass wall. Fifteen platinum serpentine microheaters, bonded on the Pyrex glass wall, were used to measure local instantaneous wall temperatures. At low mass flux, a syringe pump was used to drive the subcooled water passing through the microchannel. Stable and unstable flow boiling modes in the single microchannel are identified, and flow pattern maps in terms of heat flux and mass flux as well as in term of exit vapor quality are presented respectively. It was found that unstable flow boiling occurred in the single microchannel if the exit vapor quality xe N 0.013. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction In the past few years, flow boiling in microchannels has received considerable attention because of its possible applications in the cooling of Micro-Electro Mechanical Systems and high-performance microprocessors. Using a high-speed camera, Kandlikar et al. [1] first observed that vapor plug expanded periodically in the direction opposite to the fluid flow in their flow boiling experiments in parallel minichannels with a hydraulic diameter of 1 mm. Brutin et al. [2] investigated flow boiling instabilities in a microchannel with hydraulic diameter of 889 μm, and found stable and unstable flow boiling patterns in a microchannel depending on heat flux and mass flux. Wu and Cheng [3,4] carried out a simultaneous visualization and measurement investigation on flow boiling of water in parallel silicon microchannels of trapezoidal cross-section having hydraulic diameters of 82.8 μm, 158.8 μm, and 185.6 μm respectively. They found for the first time that there existed two oscillatory flow boiling modes with large amplitudes/long periods of temperature and pressure fluctuations in microchannels: Liquid/Two-phase Alternating Flow (LTAF) and Liquid/Two-phase/Vapor Alternating Flow (LTVAF). Qu and Mudawar [5] performed a boiling experiment of water in a copperbased microchannel with a hydraulic diameter 349 μm, and found that flow interaction between neighboring microchannels in the inlet and outlet plenums played an important role in flow boiling instability. Hetsroni et al. [6] captured the rapid reversed flow of vapor bubble by a high-speed camera, and observed periodic wetting and dryout phenomenon in flow boiling in silicon parallel microchannels. Wang et al. [7] performed further experimental studies on flow boiling in
parallel silicon microchannels and presented a boiling flow pattern map in terms of heat flux versus mass flux, showing stable and unstable flow boiling regimes for a specific inlet water temperature of 35 °C. Most recently, Wang et al. [8] found that the configuration of the inlet and outlet connection of the silicon microchannels greatly affected the amplitude of flow boiling instability, and that the exit vapor quality can be used to determine stable and unstable flow boiling regimes. Most of the previous microscale flow boiling experiments were conducted in parallel silicon microchannels, where flow interaction in the inlet/outlet plenums and axial heat conduction were significant. Very few experiments were conducted for flow boiling in a single microchannel because of the difficulties in accurate measurements of mass flux and heat flux. Recently, Huh et al. [9] investigated flow boiling of water in a single microchannel having hydraulic diameters of 103 and 133 µm, with six platinum microheaters fabricated on the bottom wall. They confirmed that very long period and large amplitude fluctuations of wall temperature and pressure drop existed in their microchannel flow boiling experiment. In this paper, we have performed further simultaneous visualization and measurement study on flow boiling in a single microchannel with fifteen platinum microheaters fabricated on a Pyrex glass wall. The effects of inlet temperature, mass flux and heat flux on flow boiling instability are investigated, and the stable and unstable flow boiling regions in the single microchannel are identified. 2. Description of the experiment 2.1. Experimental setup
☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (P. Cheng). 0735-1933/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2008.07.019
Fig. 1 shows the experimental apparatus and the flow circuit consisting of four major components: a working fluid loop, a test section integrated with heating units, a data acquisition system, and a
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G. Wang, P. Cheng / International Communications in Heat and Mass Transfer 35 (2008) 1229–1234
Fig. 1. Experimental test loop.
visualization system. The deionized water, being pushed either by a high-pressure tank (at high mass flux) or a syringe pump (at low mass flux), flowed successively through a degassing unit, a constant temperature bath, a 0.5 µm-filter, a needle valve (to regulate the mass flux pushed by the high-pressure tank) and finally the test section. After the test section, water was collected in a container at atmospheric pressure. Fifteen microheaters with response time of 0.001 s were connected with fifteen DC power supplies separately. Fifteen precise resistors (R = 50 Ω) were connected in series with microheaters, and the voltages across these resistors, Vr, were measured. The current delivered to the microheater was calculated from I = Vr / R, and the resistance Rh of the microheater was calculated according to Rh = Vh / I, where Vh was the input voltage to the microheater. The average temperature of the microheater was determined from Rh according to the relationship between resistance and temperature of platinum microheaters. Two pressure transducers were used to measure water pressures (pin and pout) at inlet and outlet of the microchannel. Two type-T thermocouples were used to measure inlet and outlet water temperature (Tin and Tout). All pressure, temperature and voltage (Vr and Vh) signals were collected by NI highspeed data acquisition system. A microscope and a high-speed camera were used to record flow boiling patterns in the microchannel.
width and 30 mm in length, which was bonded on the bottom Pyrex glass wall having 276 μm in width and 40 mm in length. 2.3. Data reduction The total heat flux supplied to the microheater and the effective heat flux absorbed by the fluid were calculated according to: q ¼ Vh I=A and qeff ¼ q−qloss
where Vh and I are the voltage and current of the microchannel, and A is the area of the microheater. The conduction heat loss qloss from the Pyrex plate was determined prior to the boiling experiment. The thermodynamic vapor quality at the exit, xe, was computed according to: xe ¼
he −hf hin −hf A ¼ þ Bo Ac hfg hfg
ð2Þ
where hf is the saturated liquid enthalpy, hfg the latent heat of evaporation (both were evaluated at the exit pressure), he the fluid enthalpy at the exit, hin the inlet fluid enthalpy, Ac the cross-sectional area of the microchannel, and Bo =q /Ghfg the boiling number with q and G being the heat flux and mass flux respectively. Eq. (2) shows that the exit vapor quality xe has included the effects of q /G (or Bo), the type of
2.2. Test section The test section included three parts: a top wall, a pair of side-walls and a bottom wall. The top of the microchannel was sealed by a thin Pyrex glass plate. The two side-walls were etched in a 〈100〉 silicon substrate. The bottom wall of the microchannel was made of Pyrex glass on which fifteen separately microheaters (numbered from #1 to #15 in the flow direction) were fabricated. The two silicon side-walls, bonded by the top and the bottom Pyrex glass plates, constructed a microchannel with a length of 40 mm. The top width, bottom width, and depth of the trapezoidal microchannel were 427 μm, 276 μm, and 107 μm, respectively, and the microchannel had a hydraulic diameter of 155 μm. Fifteen identical microheaters, fabricated by MEMS techniques with a typical lift-off metallization application, were located between 5 mm and 35 mm from the entrance at the bottom Pyrex glass wall of the microchannel. One of the fifteen serpentine microheaters is shown in Fig. 2a. The detailed dimensions of a section of the microheater are shown in Fig. 2b. The thin film platinum heater was 20 μm in width with each strip being 10 μm apart from its neighbors. Thus, the heater with an area of 0.28 mm2, covered approximately 70% of the Pyrex glass area. The fifteen microheaters occupied a total area of 200 μm in
ð1a; bÞ
Fig. 2. Photos of a microheater.
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660 kW/m2 and 780 kW/m2) with decreasing mass fluxes and with inlet water temperatures of 20 °C and 40 °C, respectively in a microchannel having a cross-sectional area of 0.0037 mm2 with a microheater having an area of 0.28 mm2. Fig. 3a shows stable and unstable flow boiling regimes in the microchannel in terms of heat flux versus mass flux at two different inlet water temperatures. The two inclined straight lines are q/G=2.70 kJ/ kg (Bo=0.001196) for water with inlet temperature Tin =20 °C and q/G= 2.08 kJ/kg (Bo=0.0009217) for water with inlet temperature Tin =40 °C. These two straight lines estimated from experimental data points divide the flow boiling map into stable flow boiling regime in the bottom half and unstable flow boiling regime in the upper half. This implies that the stable and unstable flow boiling regimes depend on two parameters of q/G (or Bo) and Tin for water in a given microchannel with a specific microheater. The data presented in Fig. 3a is re-plotted in Fig. 3b in terms of the exit vapor quality, which shows that the vertical line xe =0.013 divides the stable flow boiling regime on left and unstable flow boiling regime on the right. This critical value of the exit vapor quality is independent of inlet fluid temperature, heat and mass fluxes, the geometries of the microchannel and the microheater, as well as the physical properties of the fluid. 3.2. Stable flow boiling in a single microchannel
Fig. 3. Stable and unstable flow boiling regions in a single microchannel with Dh = 155 μm.
working fluid, the inlet fluid temperature, the cross-sectional area of the microchannel and the area of the microheater. 3. Results and discussion In this section, two sets of parameters are used to identify stable and unstable flow boiling regimes in the microchannel. Then, the flow patterns, temperature and pressure variations in the stable and unstable flow boiling modes in the microchannel are presented. 3.1. Flow boiling map in a single microchannel To investigate effects of heat and mass fluxes and inlet water temperature on flow boiling instability, experiments were carried out at five different heat fluxes (370 kW/m2, 480 kW/m2, 580 kW/m2,
An experimental run at the condition of qeff = 565.4 kW/m2, G = 220.8 kg/m2 s and Tin = 20 °C (xe = 0.004) was carried out. Fig. 4a shows that subcooled water existed near the entrance of the microheater #1. As subcooled water was heated up, isolated bubbles grew and detached to move downstream over microheater #8 (Fig. 4b). When the bubbles were flushed away, isolated bubbles formed a large bubble due to bubble coalescence over microheater #12 (Fig. 4c), and elongated bubbly/slug flow was formed near outlet section over microheater #15 (Fig. 4d). As shown in Fig. 5a and b, all temperature (Tin, Tout, and Tw1 − Tw15) and pressure (pin and pout) measurements remained constant with time in this experimental run, implying stable flow boiling mode. As seen from Fig. 5a the axial wall temperature increased along the flow direction from the entrance (Tw1) to the outlet (Tw15) under constant heat flux condition. 3.3. Unstable flow boiling in a single microchannel Fig. 6 gives the photos on the evolution of flow patterns in an unstable flow boiling mode during a cycle near the outlet (i.e., microheater #15) under the condition of q = 567.4 kW/m2, G =117.8 kg/m2 s and Tin = 20 °C (i.e., xe =0.139). In the bubbly flow shown in Fig. 6a (with t0 = 253.4 s as the reference time), isolated bubbles coalesced into large bubbles which were flushed downstream. When a bubble grew to touch the heated walls or the top Pyrex glass cover at 19.3 s later, it began to expand toward upstream and/or downstream directions (due to confinement of the walls) and formed elongated bubbly/slug flow as shown in Fig. 6b. This flow pattern behaved like an annular flow, in which the thin liquid film evaporated between the vapor core and heating wall. When long bubbles occupied the channel, they blocked the two-phase flow and caused higher pressure drop. At 35.5 s later, the upstream vapor plug broke
Fig. 4. Photos of flow patterns over four microheaters in stable flow boiling in a single microchannel with Dh = 155 μm at qeff = 565.4 kW/m2, G = 220.8 kg/m2 s, and Tin = 20 °C (xe = 0.004).
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Fig. 5. Measurements of inlet/outlet water as well as wall temperatures and inlet/outlet pressures in a microchannel with Dh = 155 μm at qeff = 565.4 kW/m2, G = 220.8 kg/m2 s, and Tin = 20 °C (xe = 0.004).
through the liquid front, reaching the long bubble downstream to form a semi-annular flow as shown in Fig. 6c. In this flow pattern, local dryout occurred due to the depletion of liquid film between vapor core and
heating wall. Fig. 6d shows that annular/mist flow (alternating dryout and rewetting phenomena) occurred in the microchannel at 66.3 s later. The appearance of the annular/mist flow was a signal of the imminent burnout of the microheater. Simultaneous visualization and temperature data acquisition confirmed that average wall temperature in bubbly flow or elongated flow was lower than that in semi-annular flow and annular/ mist flow. Fig. 7 gives temporal variations of wall temperature near the outlet section (microheater #15) at approximately the same heat flux of qeff = 570 kW/m2 and at Tin = 20 °C for three decreasing mass fluxes of 198.7 kg/m2 s, 161.9 kg/m2 s, and 117.8 kg/m2 s. Based on visualization and data acquisition, different flow patterns at various times near the outlet are also indicated in Fig. 7. At G = 198.7 kg/m2 s (xe = 0.023), the flow boiling mode near the outlet was alternating bubbly flow (100–105 °C) and elongated bubbly/slug flow (105–120 °C) with a period of 179 s (Fig. 7a); As the mass flux was decreased to G = 161.9 kg/m2 s (xe = 0.062), alternating bubbly flow (100–110 °C), elongated bubbly/slug flow (110–130 °C) and semi-annular flow (approximately 160 °C) appeared at the outlet with a period of 185 s (Fig. 7b); When the mass flux was decreased to G = 117.8 kg/m2 s (xe = 0.139), the flow boiling mode near the outlet was alternating bubbly flow (approximately 110 °C), elongated bubbly/slug flow (120–150 °C), semi-annular flow (approximately 160 °C) and annular/ mist flow (>160 °C) with a period of 200 s (Fig. 7c). The three figures shown in Fig. 7 are combined and presented in Fig. 8a for comparison purposes. It can be seen from this figure that lower mass flux resulted in higher wall temperature, larger amplitude of temperature oscillation, and longer oscillation period. The corresponding three sets of pressure-drop data are presented in Fig. 8b, which shows that the lower mass flux resulted in higher pressure drop and larger amplitude of pressure-drop oscillation. The fact that decreasing mass fluxes corresponding to higher pressure drops implies that the three experimental runs were performed in the negative sloping portion of the pressure drop versus flow rate curve. Further decrease in mass flux from 117.8 kg/m2 s may lead to large scale dryout at the heat flux of qeff =570 kW/m2. As discussed by Boure et al. [10], the pressure-drop instability is characterized by long period fluctuation of flow parameters. Since long period of pressure drop and temperature fluctuations were recorded in this experimental run, it can be concluded that these fluctuations can be classified as the pressure-drop type instability.
Fig. 6. Photos on evaluation of flow patterns near the outlet during a cycle in unstable flow boiling in a single microchannel with Dh = 155 μm at qeff = 567.4 kW/m2, G = 117.8 kg/m2 s, and Tin = 20 °C.
G. Wang, P. Cheng / International Communications in Heat and Mass Transfer 35 (2008) 1229–1234
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Fig. 8. Effects of mass flux on oscillation of wall temperature near the outlet and pressure drop in unstable flow boiling in a single microchannel with Dh = 155 μm and Tin = 20 °C at a heat flux of approximately qeff = 570 kW/m2.
Visualization shows that the flow patterns above microheater #15 (near the outlet) were alternating bubbly flow, elongated bubbly/slug flow, semi-annular flow and annular/mist flow while the flow patterns above the microheater #8 were alternating bubbly flow, elongated bubbly/slug flow and semi-annular flow. In addition, it is shown that all of the wall temperatures fluctuate in phase, which was caused by the reversed flow of vapor bubble. Visualization above microheater #1 showed that many
Fig. 7. Measurements of the wall temperatures of the microheater #15 in unstable flow boiling in a single microchannel with Dh = 155 μm at qeff = 570 kW/m2 and Tin = 20 °C.
Ding et al. [11] pointed out that the upstream compressible volume has significant effects in pressure-drop type instability. Since the amount of upstream compressible volume required for pressure-drop instability is less in a single microchannel than that in parallel microchannels of the same size, pressure-drop instability is easier to occur in a single microchannel. Fig. 9 shows the temporal variation of inlet water temperature, and wall temperatures at three different locations of the microchannel (microheaters #1, #8 and #15) at qeff =567.4 kW/m2 and G=117.8 kg/m2 s (the same condition as Fig. 7c) in the unstable flow boiling region.
Fig. 9. Oscillations of inlet water temperature and microheater #1, #8 and #15 temperatures in unstable flow boiling in a single microchannel with Dh =155 μm at qeff =567.4 kW/m2, G=117.8 kg/m2 s, and Tin =20 °C.
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fine bubbles were pushed toward the entrance of the microchannel by rapid bubble expansion in the middle section of the channel (e.g. near microheater #8), causing increase in wall temperature of microheater #1, inlet water temperature and pressure drop.
(4) The periods of temperature and pressure-drop oscillations in unstable flow boiling region are in-phase and increased as the mass flux was decreased. The oscillation periods of flow boiling instability in the single microchannel in this experiment were larger than 100 s.
4. Concluding remarks Acknowledgement In this paper, microfabrication techniques were used to fabricate a microchannel integrated with fifteen microheaters, serving also as temperature sensors. Simultaneous visualization and measurements of temperature, heat flux and mass flux have been carried out to investigate flow boiling instability of water in a single microchannel having a hydraulic diameter of 155 μm. The following conclusions can be drawn: (1) The occurrence of stable and unstable flow boiling regime in a microchannel can be determined uniquely by the exit vapor quality xe. It was found experimentally that the critical value of xe = 0.013 divides the stable and unstable flow boiling regime. This critical value is independent of inlet fluid temperature, heat flux, mass flux, the type of fluid, geometries of the microchannel and the microheater. (2) Stable flow boiling occurred in a single microchannel if xe b 0.013, where vapor bubbles generated and pushed away by the incoming subcooled liquid. Single-phase liquid flow, bubbly flow and elongated bubbly/slug flow were observed sequentially in the stable flow boiling mode. (3) Unstable flow boiling occurred in a single microchannel if xe N 0.013, where vapor plug expanded upstream resulting in oscillations of temperature and pressure. In this unstable flow boiling mode, bubbly flow, elongated bubbly/slug flow, semiannular flow and annular/mist flow were observed at different times near the outlet depending on the mass flux. Visualization and data acquisition show that different flow patterns occurred corresponding to different local wall temperatures.
This work was supported by the National Natural Science Foundation of China through Grant No. 50536010. References [1] S.G. Kandlikar, M.E. Steinke, S. Tian, L.A. Campbell, High speed photographic observation of flow boiling of water in parallel minichannels, ASME National Heat Transfer Conference, Los Angeles, CA, June 10–12, 2001. [2] D. Brutin, F. Topin, L. Tadrist, Experimental study of unsteady convective boiling in heated minichannels, Int. J. Heat Mass Transfer 46 (2003) 2957–2965. [3] H.Y. Wu, P. Cheng, Visualization and measurements of periodic boiling in silicon microchannels, Int. J. Heat Mass Transfer 46 (2003) 2603–2614. [4] H.Y. Wu, P. Cheng, Boiling instability in parallel silicon microchannels at different heat flux, Int. J. Heat Mass Transfer 47 (2004) 3631–3641. [5] W.L. Qu, I. Mudawar, Measurement and prediction of pressure drop in two-phase micro-channel heat sinks, Int. J. Heat Mass Transfer 46 (2003) 2737–2753. [6] G. Hetsroni, A. Mosyak, E. Pogrebnyak, Z. Segal, Explosive boiling of water in parallel micro-channels, Int. J. Multiph. Flow 31 (2005) 371–392. [7] G.D. Wang, P. Cheng, H.Y. Wu, Unstable and stable flow boiling in parallel microchannels and in a single microchannel, Int. J. Heat Mass Transfer 50 (2007) 4297–4310. [8] G.D. Wang, P. Cheng, A.E. Bergles, Effects of inlet/outlet configurations on flow boiling instability in parallel microchannels, Int. J. Heat Mass Transfer 51 (2008) 2267–2281. [9] C. Huh, J. Kim, M.H. Kim, Flow pattern transition instability during flow boiling in a microchannel, Int. J. Heat Mass Transfer 50 (2007) 1049–1060. [10] J.A. Boure, A.E. Bergles, L.S. Tong, Review of two-phase flow instability, Nucl. Eng. Des. 25 (1973) 165–192. [11] Y. Ding, S. Kakac, X.J. Chen, Dynamic instability of boiling two-phase flow in a single horizontal channel, Exp. Therm. Fluid Sci. 11 (1995) 327–342.
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
An experimental study of flow boiling instability in a single microchannel☆ Guodong Wang, Ping Cheng ⁎ Ministry of Education Key Laboratory of Power Machinery and Engineering, School of Mechanical and Power Engineering, Shanghai Jiaotong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Available online 9 September 2008 Keywords: Boiling instability Heat transfer Microchannels Instability Reversed flow Single microchannel
a b s t r a c t A simultaneous visualization and measurement study has been carried out to investigate stable and unstable flow boiling phenomena of deionized water in a single microchannel having a hydraulic diameter of 155 µm with a bottom Pyrex glass wall. Fifteen platinum serpentine microheaters, bonded on the Pyrex glass wall, were used to measure local instantaneous wall temperatures. At low mass flux, a syringe pump was used to drive the subcooled water passing through the microchannel. Stable and unstable flow boiling modes in the single microchannel are identified, and flow pattern maps in terms of heat flux and mass flux as well as in term of exit vapor quality are presented respectively. It was found that unstable flow boiling occurred in the single microchannel if the exit vapor quality xe N 0.013. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction In the past few years, flow boiling in microchannels has received considerable attention because of its possible applications in the cooling of Micro-Electro Mechanical Systems and high-performance microprocessors. Using a high-speed camera, Kandlikar et al. [1] first observed that vapor plug expanded periodically in the direction opposite to the fluid flow in their flow boiling experiments in parallel minichannels with a hydraulic diameter of 1 mm. Brutin et al. [2] investigated flow boiling instabilities in a microchannel with hydraulic diameter of 889 μm, and found stable and unstable flow boiling patterns in a microchannel depending on heat flux and mass flux. Wu and Cheng [3,4] carried out a simultaneous visualization and measurement investigation on flow boiling of water in parallel silicon microchannels of trapezoidal cross-section having hydraulic diameters of 82.8 μm, 158.8 μm, and 185.6 μm respectively. They found for the first time that there existed two oscillatory flow boiling modes with large amplitudes/long periods of temperature and pressure fluctuations in microchannels: Liquid/Two-phase Alternating Flow (LTAF) and Liquid/Two-phase/Vapor Alternating Flow (LTVAF). Qu and Mudawar [5] performed a boiling experiment of water in a copperbased microchannel with a hydraulic diameter 349 μm, and found that flow interaction between neighboring microchannels in the inlet and outlet plenums played an important role in flow boiling instability. Hetsroni et al. [6] captured the rapid reversed flow of vapor bubble by a high-speed camera, and observed periodic wetting and dryout phenomenon in flow boiling in silicon parallel microchannels. Wang et al. [7] performed further experimental studies on flow boiling in
parallel silicon microchannels and presented a boiling flow pattern map in terms of heat flux versus mass flux, showing stable and unstable flow boiling regimes for a specific inlet water temperature of 35 °C. Most recently, Wang et al. [8] found that the configuration of the inlet and outlet connection of the silicon microchannels greatly affected the amplitude of flow boiling instability, and that the exit vapor quality can be used to determine stable and unstable flow boiling regimes. Most of the previous microscale flow boiling experiments were conducted in parallel silicon microchannels, where flow interaction in the inlet/outlet plenums and axial heat conduction were significant. Very few experiments were conducted for flow boiling in a single microchannel because of the difficulties in accurate measurements of mass flux and heat flux. Recently, Huh et al. [9] investigated flow boiling of water in a single microchannel having hydraulic diameters of 103 and 133 µm, with six platinum microheaters fabricated on the bottom wall. They confirmed that very long period and large amplitude fluctuations of wall temperature and pressure drop existed in their microchannel flow boiling experiment. In this paper, we have performed further simultaneous visualization and measurement study on flow boiling in a single microchannel with fifteen platinum microheaters fabricated on a Pyrex glass wall. The effects of inlet temperature, mass flux and heat flux on flow boiling instability are investigated, and the stable and unstable flow boiling regions in the single microchannel are identified. 2. Description of the experiment 2.1. Experimental setup
☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (P. Cheng). 0735-1933/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2008.07.019
Fig. 1 shows the experimental apparatus and the flow circuit consisting of four major components: a working fluid loop, a test section integrated with heating units, a data acquisition system, and a
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G. Wang, P. Cheng / International Communications in Heat and Mass Transfer 35 (2008) 1229–1234
Fig. 1. Experimental test loop.
visualization system. The deionized water, being pushed either by a high-pressure tank (at high mass flux) or a syringe pump (at low mass flux), flowed successively through a degassing unit, a constant temperature bath, a 0.5 µm-filter, a needle valve (to regulate the mass flux pushed by the high-pressure tank) and finally the test section. After the test section, water was collected in a container at atmospheric pressure. Fifteen microheaters with response time of 0.001 s were connected with fifteen DC power supplies separately. Fifteen precise resistors (R = 50 Ω) were connected in series with microheaters, and the voltages across these resistors, Vr, were measured. The current delivered to the microheater was calculated from I = Vr / R, and the resistance Rh of the microheater was calculated according to Rh = Vh / I, where Vh was the input voltage to the microheater. The average temperature of the microheater was determined from Rh according to the relationship between resistance and temperature of platinum microheaters. Two pressure transducers were used to measure water pressures (pin and pout) at inlet and outlet of the microchannel. Two type-T thermocouples were used to measure inlet and outlet water temperature (Tin and Tout). All pressure, temperature and voltage (Vr and Vh) signals were collected by NI highspeed data acquisition system. A microscope and a high-speed camera were used to record flow boiling patterns in the microchannel.
width and 30 mm in length, which was bonded on the bottom Pyrex glass wall having 276 μm in width and 40 mm in length. 2.3. Data reduction The total heat flux supplied to the microheater and the effective heat flux absorbed by the fluid were calculated according to: q ¼ Vh I=A and qeff ¼ q−qloss
where Vh and I are the voltage and current of the microchannel, and A is the area of the microheater. The conduction heat loss qloss from the Pyrex plate was determined prior to the boiling experiment. The thermodynamic vapor quality at the exit, xe, was computed according to: xe ¼
he −hf hin −hf A ¼ þ Bo Ac hfg hfg
ð2Þ
where hf is the saturated liquid enthalpy, hfg the latent heat of evaporation (both were evaluated at the exit pressure), he the fluid enthalpy at the exit, hin the inlet fluid enthalpy, Ac the cross-sectional area of the microchannel, and Bo =q /Ghfg the boiling number with q and G being the heat flux and mass flux respectively. Eq. (2) shows that the exit vapor quality xe has included the effects of q /G (or Bo), the type of
2.2. Test section The test section included three parts: a top wall, a pair of side-walls and a bottom wall. The top of the microchannel was sealed by a thin Pyrex glass plate. The two side-walls were etched in a 〈100〉 silicon substrate. The bottom wall of the microchannel was made of Pyrex glass on which fifteen separately microheaters (numbered from #1 to #15 in the flow direction) were fabricated. The two silicon side-walls, bonded by the top and the bottom Pyrex glass plates, constructed a microchannel with a length of 40 mm. The top width, bottom width, and depth of the trapezoidal microchannel were 427 μm, 276 μm, and 107 μm, respectively, and the microchannel had a hydraulic diameter of 155 μm. Fifteen identical microheaters, fabricated by MEMS techniques with a typical lift-off metallization application, were located between 5 mm and 35 mm from the entrance at the bottom Pyrex glass wall of the microchannel. One of the fifteen serpentine microheaters is shown in Fig. 2a. The detailed dimensions of a section of the microheater are shown in Fig. 2b. The thin film platinum heater was 20 μm in width with each strip being 10 μm apart from its neighbors. Thus, the heater with an area of 0.28 mm2, covered approximately 70% of the Pyrex glass area. The fifteen microheaters occupied a total area of 200 μm in
ð1a; bÞ
Fig. 2. Photos of a microheater.
G. Wang, P. Cheng / International Communications in Heat and Mass Transfer 35 (2008) 1229–1234
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660 kW/m2 and 780 kW/m2) with decreasing mass fluxes and with inlet water temperatures of 20 °C and 40 °C, respectively in a microchannel having a cross-sectional area of 0.0037 mm2 with a microheater having an area of 0.28 mm2. Fig. 3a shows stable and unstable flow boiling regimes in the microchannel in terms of heat flux versus mass flux at two different inlet water temperatures. The two inclined straight lines are q/G=2.70 kJ/ kg (Bo=0.001196) for water with inlet temperature Tin =20 °C and q/G= 2.08 kJ/kg (Bo=0.0009217) for water with inlet temperature Tin =40 °C. These two straight lines estimated from experimental data points divide the flow boiling map into stable flow boiling regime in the bottom half and unstable flow boiling regime in the upper half. This implies that the stable and unstable flow boiling regimes depend on two parameters of q/G (or Bo) and Tin for water in a given microchannel with a specific microheater. The data presented in Fig. 3a is re-plotted in Fig. 3b in terms of the exit vapor quality, which shows that the vertical line xe =0.013 divides the stable flow boiling regime on left and unstable flow boiling regime on the right. This critical value of the exit vapor quality is independent of inlet fluid temperature, heat and mass fluxes, the geometries of the microchannel and the microheater, as well as the physical properties of the fluid. 3.2. Stable flow boiling in a single microchannel
Fig. 3. Stable and unstable flow boiling regions in a single microchannel with Dh = 155 μm.
working fluid, the inlet fluid temperature, the cross-sectional area of the microchannel and the area of the microheater. 3. Results and discussion In this section, two sets of parameters are used to identify stable and unstable flow boiling regimes in the microchannel. Then, the flow patterns, temperature and pressure variations in the stable and unstable flow boiling modes in the microchannel are presented. 3.1. Flow boiling map in a single microchannel To investigate effects of heat and mass fluxes and inlet water temperature on flow boiling instability, experiments were carried out at five different heat fluxes (370 kW/m2, 480 kW/m2, 580 kW/m2,
An experimental run at the condition of qeff = 565.4 kW/m2, G = 220.8 kg/m2 s and Tin = 20 °C (xe = 0.004) was carried out. Fig. 4a shows that subcooled water existed near the entrance of the microheater #1. As subcooled water was heated up, isolated bubbles grew and detached to move downstream over microheater #8 (Fig. 4b). When the bubbles were flushed away, isolated bubbles formed a large bubble due to bubble coalescence over microheater #12 (Fig. 4c), and elongated bubbly/slug flow was formed near outlet section over microheater #15 (Fig. 4d). As shown in Fig. 5a and b, all temperature (Tin, Tout, and Tw1 − Tw15) and pressure (pin and pout) measurements remained constant with time in this experimental run, implying stable flow boiling mode. As seen from Fig. 5a the axial wall temperature increased along the flow direction from the entrance (Tw1) to the outlet (Tw15) under constant heat flux condition. 3.3. Unstable flow boiling in a single microchannel Fig. 6 gives the photos on the evolution of flow patterns in an unstable flow boiling mode during a cycle near the outlet (i.e., microheater #15) under the condition of q = 567.4 kW/m2, G =117.8 kg/m2 s and Tin = 20 °C (i.e., xe =0.139). In the bubbly flow shown in Fig. 6a (with t0 = 253.4 s as the reference time), isolated bubbles coalesced into large bubbles which were flushed downstream. When a bubble grew to touch the heated walls or the top Pyrex glass cover at 19.3 s later, it began to expand toward upstream and/or downstream directions (due to confinement of the walls) and formed elongated bubbly/slug flow as shown in Fig. 6b. This flow pattern behaved like an annular flow, in which the thin liquid film evaporated between the vapor core and heating wall. When long bubbles occupied the channel, they blocked the two-phase flow and caused higher pressure drop. At 35.5 s later, the upstream vapor plug broke
Fig. 4. Photos of flow patterns over four microheaters in stable flow boiling in a single microchannel with Dh = 155 μm at qeff = 565.4 kW/m2, G = 220.8 kg/m2 s, and Tin = 20 °C (xe = 0.004).
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Fig. 5. Measurements of inlet/outlet water as well as wall temperatures and inlet/outlet pressures in a microchannel with Dh = 155 μm at qeff = 565.4 kW/m2, G = 220.8 kg/m2 s, and Tin = 20 °C (xe = 0.004).
through the liquid front, reaching the long bubble downstream to form a semi-annular flow as shown in Fig. 6c. In this flow pattern, local dryout occurred due to the depletion of liquid film between vapor core and
heating wall. Fig. 6d shows that annular/mist flow (alternating dryout and rewetting phenomena) occurred in the microchannel at 66.3 s later. The appearance of the annular/mist flow was a signal of the imminent burnout of the microheater. Simultaneous visualization and temperature data acquisition confirmed that average wall temperature in bubbly flow or elongated flow was lower than that in semi-annular flow and annular/ mist flow. Fig. 7 gives temporal variations of wall temperature near the outlet section (microheater #15) at approximately the same heat flux of qeff = 570 kW/m2 and at Tin = 20 °C for three decreasing mass fluxes of 198.7 kg/m2 s, 161.9 kg/m2 s, and 117.8 kg/m2 s. Based on visualization and data acquisition, different flow patterns at various times near the outlet are also indicated in Fig. 7. At G = 198.7 kg/m2 s (xe = 0.023), the flow boiling mode near the outlet was alternating bubbly flow (100–105 °C) and elongated bubbly/slug flow (105–120 °C) with a period of 179 s (Fig. 7a); As the mass flux was decreased to G = 161.9 kg/m2 s (xe = 0.062), alternating bubbly flow (100–110 °C), elongated bubbly/slug flow (110–130 °C) and semi-annular flow (approximately 160 °C) appeared at the outlet with a period of 185 s (Fig. 7b); When the mass flux was decreased to G = 117.8 kg/m2 s (xe = 0.139), the flow boiling mode near the outlet was alternating bubbly flow (approximately 110 °C), elongated bubbly/slug flow (120–150 °C), semi-annular flow (approximately 160 °C) and annular/ mist flow (>160 °C) with a period of 200 s (Fig. 7c). The three figures shown in Fig. 7 are combined and presented in Fig. 8a for comparison purposes. It can be seen from this figure that lower mass flux resulted in higher wall temperature, larger amplitude of temperature oscillation, and longer oscillation period. The corresponding three sets of pressure-drop data are presented in Fig. 8b, which shows that the lower mass flux resulted in higher pressure drop and larger amplitude of pressure-drop oscillation. The fact that decreasing mass fluxes corresponding to higher pressure drops implies that the three experimental runs were performed in the negative sloping portion of the pressure drop versus flow rate curve. Further decrease in mass flux from 117.8 kg/m2 s may lead to large scale dryout at the heat flux of qeff =570 kW/m2. As discussed by Boure et al. [10], the pressure-drop instability is characterized by long period fluctuation of flow parameters. Since long period of pressure drop and temperature fluctuations were recorded in this experimental run, it can be concluded that these fluctuations can be classified as the pressure-drop type instability.
Fig. 6. Photos on evaluation of flow patterns near the outlet during a cycle in unstable flow boiling in a single microchannel with Dh = 155 μm at qeff = 567.4 kW/m2, G = 117.8 kg/m2 s, and Tin = 20 °C.
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Fig. 8. Effects of mass flux on oscillation of wall temperature near the outlet and pressure drop in unstable flow boiling in a single microchannel with Dh = 155 μm and Tin = 20 °C at a heat flux of approximately qeff = 570 kW/m2.
Visualization shows that the flow patterns above microheater #15 (near the outlet) were alternating bubbly flow, elongated bubbly/slug flow, semi-annular flow and annular/mist flow while the flow patterns above the microheater #8 were alternating bubbly flow, elongated bubbly/slug flow and semi-annular flow. In addition, it is shown that all of the wall temperatures fluctuate in phase, which was caused by the reversed flow of vapor bubble. Visualization above microheater #1 showed that many
Fig. 7. Measurements of the wall temperatures of the microheater #15 in unstable flow boiling in a single microchannel with Dh = 155 μm at qeff = 570 kW/m2 and Tin = 20 °C.
Ding et al. [11] pointed out that the upstream compressible volume has significant effects in pressure-drop type instability. Since the amount of upstream compressible volume required for pressure-drop instability is less in a single microchannel than that in parallel microchannels of the same size, pressure-drop instability is easier to occur in a single microchannel. Fig. 9 shows the temporal variation of inlet water temperature, and wall temperatures at three different locations of the microchannel (microheaters #1, #8 and #15) at qeff =567.4 kW/m2 and G=117.8 kg/m2 s (the same condition as Fig. 7c) in the unstable flow boiling region.
Fig. 9. Oscillations of inlet water temperature and microheater #1, #8 and #15 temperatures in unstable flow boiling in a single microchannel with Dh =155 μm at qeff =567.4 kW/m2, G=117.8 kg/m2 s, and Tin =20 °C.
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fine bubbles were pushed toward the entrance of the microchannel by rapid bubble expansion in the middle section of the channel (e.g. near microheater #8), causing increase in wall temperature of microheater #1, inlet water temperature and pressure drop.
(4) The periods of temperature and pressure-drop oscillations in unstable flow boiling region are in-phase and increased as the mass flux was decreased. The oscillation periods of flow boiling instability in the single microchannel in this experiment were larger than 100 s.
4. Concluding remarks Acknowledgement In this paper, microfabrication techniques were used to fabricate a microchannel integrated with fifteen microheaters, serving also as temperature sensors. Simultaneous visualization and measurements of temperature, heat flux and mass flux have been carried out to investigate flow boiling instability of water in a single microchannel having a hydraulic diameter of 155 μm. The following conclusions can be drawn: (1) The occurrence of stable and unstable flow boiling regime in a microchannel can be determined uniquely by the exit vapor quality xe. It was found experimentally that the critical value of xe = 0.013 divides the stable and unstable flow boiling regime. This critical value is independent of inlet fluid temperature, heat flux, mass flux, the type of fluid, geometries of the microchannel and the microheater. (2) Stable flow boiling occurred in a single microchannel if xe b 0.013, where vapor bubbles generated and pushed away by the incoming subcooled liquid. Single-phase liquid flow, bubbly flow and elongated bubbly/slug flow were observed sequentially in the stable flow boiling mode. (3) Unstable flow boiling occurred in a single microchannel if xe N 0.013, where vapor plug expanded upstream resulting in oscillations of temperature and pressure. In this unstable flow boiling mode, bubbly flow, elongated bubbly/slug flow, semiannular flow and annular/mist flow were observed at different times near the outlet depending on the mass flux. Visualization and data acquisition show that different flow patterns occurred corresponding to different local wall temperatures.
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