Flow patterns and heat transfer mechanisms during flow boiling over open microchannels in tapered manifold (OMM)

Flow patterns and heat transfer mechanisms during flow boiling over open microchannels in tapered manifold (OMM)

International Journal of Heat and Mass Transfer 89 (2015) 494–504 Contents lists available at ScienceDirect International Journal of Heat and Mass T...

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International Journal of Heat and Mass Transfer 89 (2015) 494–504

Contents lists available at ScienceDirect

International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt

Flow patterns and heat transfer mechanisms during flow boiling over open microchannels in tapered manifold (OMM) Ankit Kalani a, Satish G. Kandlikar a,b,⇑ a b

Microsystems Engineering, Rochester Institute of Technology, 168 Lomb Memorial Dr., Rochester, NY 14623, USA Mechanical Engineering, Rochester Institute of Technology, 76 Lomb Memorial Dr., Rochester, NY 14623, USA

a r t i c l e

i n f o

Article history: Received 16 February 2015 Received in revised form 15 May 2015 Accepted 15 May 2015

Keywords: Heat transfer mechanism High speed visualization OMM Flow patterns Tapered manifold Flow boiling heat transfer

a b s t r a c t The ever increasing demand of high heat removal from compact form factor devices has generated considerable interest in advanced thermal management techniques. Flow boiling in microchannels has the ability to provide high heat dissipation due to the utilization of the latent heat of vaporization, while maintaining a uniform coolant temperature. Recently, a number of studies have introduced variable flow cross-sectional area to augment the thermal performance of microchannels. The open microchannel with manifold (OMM) configuration provides stable high heat transfer performance with very low pressure drop. In the current work, high speed images are obtained to gain an insight into the nucleating bubble behavior and flow patterns at high heat fluxes including critical heat flux (CHF). The flow patterns are plotted as a function of superficial gas and liquid velocity. The resulting map indicates significant departure from the earlier work on macroscale tubes and confined microchannels. A mechanistic description of the heat transfer mechanism is also presented and the underlying differences between flow boiling in closed microchannels and open microchannels with tapered manifold configuration are highlighted. Furthermore, bubble ebullition cycle in pool boiling is compared with the tapered geometry utilizing plain and microchannel surfaces. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Heat dissipation rate per unit device area has greatly increased due to the miniaturization of electronics [1]. As heat fluxes in medical, aerospace and defense related electronics equipment continue to increase, two-phase cooling is looked at as an attractive solution with a relatively small increase in the surface temperature. Flow boiling in microchannels has been studied as one of the cooling techniques to meet the current thermal requirements [2]. It is considered to be an attractive option due to its small hydraulic diameter, uniform temperature control, and compact design. However, the efficiency of the flow boiling technique in microchannels has been severely impacted due to early CHF [3] and flow instability [4]. For flow instabilities in microchannels, rapid/explosive bubble growth was seen as one of the main reasons. Hsu’s criterion [5] provides the required surface cavity radius for bubble nucleation. The bubble growth rate in a microchannel depends on the local flow conditions and wall superheat, and the ⇑ Corresponding author at: Mechanical Engineering, Rochester Institute of Technology, 76 Lomb Memorial Dr., Rochester, NY 14623, USA. Tel.: +1 (585) 475 6728; fax: +1 (585) 475 6879. E-mail addresses: [email protected] (A. Kalani), [email protected] (S.G. Kandlikar). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.05.070 0017-9310/Ó 2015 Elsevier Ltd. All rights reserved.

reasons for the explosive bubble growth were explained by Kandlikar [6]. The single-phase heat transfer coefficient in microchannel is high, and provides a superheated liquid state within the entire microchannel cross-sectional area. The bubble upon nucleating from the cavity experiences a highly superheated liquid environment causing it to rapidly expand in both upstream and downstream directions. For small channels, bubble nucleation causes local pressure variation along the flow path as seen from Fig. 1. The maximum pressure in the bubble can exceed the inlet pressure (Pv > Pin) due to the high wall temperature. This causes the bubble to overcome the incoming liquid inertia and travel in the upstream direction causing flow reversal. When the bubble expands, the pressure decreases and enters a relaxation period as seen in Fig. 2 [7]. During this relaxation period, the pressure inside the bubble (Pv) reduces and drops below the inlet pressure (Pin), therein reversing the upstream movement of the bubble and flowing in the direction of the fluid flow. This unstable flow causes severe pressure and temperature fluctuation and in some cases initiates an early CHF. Previous research efforts in this area have focused on overcoming these challenges by using various structures, such as inlet restrictors [8], artificial nucleation sites [9], reentrant cavities [10], vapor venting [11] and increasing flow cross-sectional area [12,13]. Further details

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Fig. 1. Pressure variation inside a microchannel during bubble nucleation, adapted from [6].


two types of flow patterns, namely annular and dryout in their study. Kandlikar [17] studied the heat transfer mechanisms in microchannels, focusing on flow instability and two-phase flow patterns (slug flow, annular flow, churn flow and dryout condition). Zhang et al. [18] observed nucleate boiling and eruption boiling in their single microchannel study. They also found the boiling mechanism to be strongly dependent on the wall surface roughness. Chen and Garimella [19] observed bubbly and slug flow at low heat fluxes. At higher heat fluxes, the authors observed annular and churn flow in the downstream section and flow reversal near the microchannel inlet. Harirchian and Garimella [20] studied the effect of various parameters on flow boiling regimes. The authors reported that the flow regimes for microchannels with 400 lm (width) and greater were similar, while microchannels with width less than 400 lm showed different flow regimes. Balasubramanian et al. [12] used a stepped microchannel in their investigation and observed bubbly, intermittent, and annular flow regimes. Recently, Tamanna and Lee [21] used expanding silicon microgap heat sink to study the bubble mechanism in their geometry through high speed visualization. Excellent reviews discussing the flow patterns for different flow conditions and other aspects of microchannel flow boiling are available in literature [22–24]. In this work, flow patterns and heat transfer mechanisms of an open microchannel with tapered manifold are investigated. Various flow patterns are observed and their transitions with increasing heat flux are described. The underlying mechanisms of bubble nucleation, growth and departure are explored through high speed visualization. Furthermore, closed microchannel and open microchannel geometries are compared via bubble dynamics through high speed image sequences. Plain surface bubble ebullition cycle and flow conditions leading to CHF in the OMM geometry are also discussed. 2. Experimental setup

Fig. 2. Pressure variation inside a microchannel during bubble expansion, adapted from [7].

on these techniques that provide enhanced heat transfer and reduced flow instability have been addressed in a previous publication by the authors [14]. The open microchannel with manifold (OMM) geometry was introduced by Kandlikar et al. [15] to provide a stable, low pressure drop and high performance system. This geometry provides additional flow area over the microchannel (manifold region) which assists in removing the generated vapor without an excessive pressure drop. Tapered manifold was used to provide gradual area increase downstream thereby reducing flow resistance and increasing flow stability. High speed flow visualizations have been conducted by various researchers under different parameters to study various flow patterns in their systems. For example, Hetsroni et al. [16] observed

Fig. 3 shows the schematic representation of the test loop used in the flow boiling study with OMM geometry. The experimental setup was designed and fabricated to study flow boiling with OMM geometry with high speed visualization capabilities. The setup details can be obtained from a previous publication [9]. Briefly, distilled water was degassed in a pressure canner using a hot plate to remove dissolved gases following the procedure recommended by Steinke and Kandlikar [25]. A MicropumpÒ was used to circulate the liquid. The flow rate for a given test run was set using a rotameter. An inline heater with a PID controller was used to provide the desired inlet temperature for the system. The exit liquid– vapor mixture from the test section was returned to the pressure canner. The test section consisted of a microchannel chip with a channel depth of 205 lm, width of 250 lm, and fin width of 145 lm. A fixed inlet height of 127 lm above the microchannel surface was used for all test runs along with a taper gradient of 4%, resulting in an increase in the manifold height by 400 lm over a flow length of 10,000 lm at the exit section. Three mass fluxes of 196, 393 and 688 kg/m2s were studied. Further details regarding the tapered manifold, heat flux and pressure drop calculations can be obtained from an earlier publication [9]. 3. High speed visualization High speed visualization was accomplished with a Photron 1024 Fastcam CMOS camera and a 150 mm Nikon lens. Additional light required for imaging was provided using a Dolan-Jenner Fiber-lite MH-100 metal halide Machine Vision illuminator lamp. The polysulfone manifold block was polished to a transparent finish to facilitate visualization. The images were taken


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Fig. 3. (a and b) Schematic of the flow boiling experimental loop and the OMM geometry.

at 3000–10,000 frames per second (fps). Experiments were performed at heat fluxes in the range 50–500 W/cm2 and an inlet subcooling of 10 °C were employed. High speed visualization of the boiling phenomena was captured to understand the flow patterns. Bubble nucleation, growth, departure and CHF conditions are all important in providing an insight into the underlying heat transfer mechanisms. The following sections describe various details of the flow patterns and some of the specific features of the bubble ebullition cycle that were noted. 3.1. Flow patterns The flow patterns obtained with the microchannel chip and the tapered manifold are discussed in this section. These patterns were identified through high speed images obtained at 3000 fps at a constant mass flux of 393 kg/m2s. They can be broadly classified into five major flow regimes – bubbly, slug, intermittent slug/ bubbly, annular, and inverted annular (post-CHF) flows. Fig. 4(a) shows the first regime, bubbly flow, at a low heat flux (100 W/cm2). Bubbles nucleate from various nucleation cavities in the microchannel (base of the channels), grow to the size of the channel width and expand onto the fin tops. Further expansion was observed on the fin tops in the manifold region before the eventual departure in the flow direction. As the heat flux increases, more nucleation sites get activated and further expansion of bubbles also takes place. Slug flow as seen in Fig. 4(b) forms the second flow regime. In this regime, the bubble expanded on top of the fin and coalesced with other expanding bubbles in the manifold

region. Intermittent slug/bubbly flow (Fig. 4(c)) was observed as the next flow pattern with an increase in heat flux (200 W/cm2). A number of nucleation sites were active and large vapor bubbles were observed downstream. Both the liquid and vapor phases were distributed over the active area as seen from the figure. In annular flow, the manifold region was mostly comprised of vapor while the channels were filled with liquid. The nucleating bubbles inside the channels fed onto the existing vapor bubbles on the fin tops (further discussed in the mechanism section). At higher heat fluxes (>300 W/cm2), the annular regime showed stable flow boiling as seen in (Fig. 4(d)). In this regime, the liquid filled the channels and the manifold region was mainly occupied by the vapor. As the heat flux further increases, the liquid film in the channel decreases in thickness. Inverted annular regime was observed once the system reached CHF. This flow pattern, as the name suggests, was the opposite of annular flow. The channels were occupied by the vapor phase and the manifold region was filled with liquid (Fig. 2(e)). This flow regime causes significant increase in the surface temperature and adversely affects the heat transfer performance. The high temperature associated with CHF could also lead to structural damage to the components. The flow patterns described in the preceding paragraphs were observed in the tapered manifold and open microchannel geometry at a fixed flow rate. Further descriptions on each of the flow patterns along with the image sequences are provided in the following sections to explain the heat transfer mechanisms. However, a flow pattern map is introduced first to show the effect of flow parameters on the two-phase flow morphology.

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Fig. 4. Flow regimes for tapered manifold and microchannel geometry (a) Bubbly flow, (b) Slug flow, (c) Intermittent slug/bubbly flow, (d) Annular flow and (e) Inverted annular flow (CHF).

3.2. Flow pattern map Effect of flow rate on flow regimes were further explored through flow pattern maps. The flow patterns for the three mass fluxes tested were plotted with liquid and vapor superficial velocities as co-ordinates as seen in literature. The liquid (jl) and vapor (jg) superficial velocities were given by:

jl ¼

jg ¼

Gð1  xÞ

ql GðxÞ




where G is the mass flux, x the exit quality, and ql and qg are the liquid and vapor densities respectively.

Fig. 5 shows the flow pattern map of microchannel with 4% tapered manifold for three mass fluxes. The bubbly flow regime occurs at low liquid and vapor superficial velocities for all mass fluxes. The transition from bubbly to slug flow for a higher mass flux occurs at a higher jg, showing the increase in the bubbly flow pattern range. For the lower mass flux, a quick transition was observed form bubbly to slug flow. This is due to the extra space above the microchannel and the gradually increasing cross-sectional area. The low liquid inertia due to the low flow rate allows the bubble to expand in the downstream direction after nucleating. Slug flow and intermittent regime occurred at a higher exit quality for the lower mass fluxes. The transition to annular flow occurs at higher superficial vapor velocity with the increase in the liquid superficial velocity. The Taitel and Dukler [27] map has often been used in literature for better understanding of the flow regimes. The data points


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3.3. Bubble nucleation mechanism in bubbly and slug flow

Fig. 5. Flow pattern map on superficial vapor and liquid velocity (jg and jl) coordinates with three mass fluxes of 196, 393 and 688 kg/m2s over a heat flux range from 50–500 W/cm2.

obtained in the current study were plotted on these maps as shown in Fig. 6. The flow pattern map showed majority of the data points in the annular regime, and some in and around the intermittent regime. This is however not true with the observations from the current experiments. Moreover, no data points were present in the bubbly flow regime in the map. While, during the experimental test runs bubbly flow was observed at low heat fluxes for all three flow rates. Similar results were reported by other researchers using the increased flow cross-sectional area geometries [12]. The Taitel and Dukler flow regime map was developed for adiabatic conditions and showed good prediction in the earlier works for macroscale tubes. The departure from this map with the current data could be due to the complex geometry involving open microchannels and a tapered manifold. Further study is warranted to come up with a comprehensive flow pattern map for tapered geometries. Specifically, the effect of taper and heat flux needs to be explored with a concurrent relation with the heat transfer and pressure drop performance. The present map however indicates significant differences between the tapered manifold configuration and the established microscale and macroscale flow pattern maps.

In traditional microchannels, the channels are covered with a cover plate (top surface) and under stable boiling conditions, bubbles nucleate in the channel and grow downstream in the flow direction. The expansion of the bubbles gives rise to the well-known elongated bubble flow pattern. However, at higher heat fluxes a nucleating bubble grows in both upstream and downstream directions [14]. This leads to explosive bubble growth and in turn to flow instability. By providing a tapered manifold gap above the microchannels, bubbles have room to grow away from the microchannels into the manifold and depart from the microchannel region without any resistance. The taper introduces a pressure recovery term and encourages the bubble to flow in the downstream direction due to a lower flow resistance as compared to the upstream direction [26]. This avoids flow instabilities due to flow reversal [9]. Detailed images of specific bubble nucleation and growth phenomena were further investigated. The mass flux is kept constant at G = 393 kg/m2s. Fig. 7 shows a nucleating bubble sequence at a relatively low heat flux of 100 W/cm2. The flow direction was from top to bottom as indicated by the white arrow. For a clearer understanding of the nucleation phenomena, the images for Figs. 7 and 8 were taken at 10,000 fps. The left image for Fig. 7(a) shows bubble nucleation and the right side shows a schematic representation of the image for depicting the mechanism. The bubbles nucleate from the bottom surface of the microchannel. The individual frames of other videos with varying taper showed similar bubble nucleation at the base of the channel. Fig. 7(b) shows the expansion of the bubble on the fin top, while a new bubble nucleates from the same site. Fig. 7(c) shows further growth of the bubble in the manifold region as nucleation continues in the channel. Fig. 6 shows a sequence of images that provide an insight into the underlying mechanism of bubble nucleation in the OMM geometry. Fig. 8(a) shows a bubble nucleating at the base of the microchannel and another large bubble passing over the microchannels in the manifold region. When the bubble in the manifold region overlaps the nucleating bubble, nucleation continues to occur in other regions of the microchannel as seen in Fig. 8(b). The individual image shows a number of bubbles present in the channel region while vapor occupies the manifold on top. This indicates that the channels are filled with liquid even when vapor is present in the manifold region. Due to the presence of liquid in the channels, the active nucleation sites continue bubble nucleation, growth and departure. Fig. 8(c) and (d) show a bubble growing and eventually bursting underneath the bubble in the manifold region. Once the bubble covering the nucleation site departs as shown in Fig. 8(e), normal bubble nucleation resumes. The individual frames from Fig. 8 show that the large vapor remains in the manifold region, while allowing the channel region to maintain liquid flow. This fundamental pattern was seen to exist at higher heat fluxes also. 3.4. Intermittent slug/bubbly flow

Fig. 6. Taitel–Dukler flow pattern map with experimental data points.

Fig. 9 shows an intermittent slug and bubbly flow pattern in the OMM geometry. The image shows the entire open microchannel surface at a moderate heat flux of 200 W/cm2. In the figure, different phenomena illustrated through individual frames from Figs. 7 and 8 are seen. At various locations (center marked in the figure), several departed bubbles are seen in the manifold region. Bubble growth and nucleation are also observed in the channel region as shown in Fig. 7. Similar to Fig. 8, bubble nucleation was observed at the base of the microchannel while vapor was present at the top in the manifold region. Bubble growth is also seen in the channel underneath the vapor indicating that the channels are liquid

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Fig. 7. (a–c) Successive images of bubble nucleation, growth and departure at low heat fluxes.

filled. Hence the mechanism at higher heat fluxes is seen to be similar to that at lower heat fluxes for bubble nucleation, growth and departure. At still higher heat fluxes, the entire manifold area was covered with vapor as seen in Fig. 10(a). The channels were flooded with water; hence in the next frame Fig. 10(b) we see bubble nucleation and growth in this liquid present underneath the vapor blanket. The extra flow cross-sectional area available in the flow direction due to taper allows bubbles to expand while leaving the channels filled with oncoming liquid. The pressure recovery induced by the area enlargement has been identified by Kalani and Kandlikar [26] as a major factor in reducing the overall pressure drop and improving the flow stability. This mechanism also helps in extending CHF due to stable operation. 3.5. Annular flow Fig. 11(a) shows a high speed image of the annular flow regime. Similar to intermittent regime (Fig. 9), bubble nucleation occurs in the channel region. However, the majority of nucleation occurs at the section close to the inlet (upstream). The channels are liquid filled with frequent nucleation activity in the downstream region.

The regime shows a high rate of bubble nucleation and departure. At heat fluxes of >300 W/cm2, this regime shows highly stable boiling with a similar bubble ebullition cycle as explained through the earlier figures. Fig. 11(b) shows a transition flow pattern between the annular and the inverted annular flow regime. In the image, bubble nucleation is observed near the inlet side of the microchannel chip in the channel region. In the downstream section, no bubble nucleation is seen from the channels and occasional dry spots are observed. 3.6. Critical heat flux Critical heat flux limits the thermal performance of a system and occurs when the vapor film covers the entire heated surface. This vapor film does not allow liquid rewetting of the surface which leads to extremely high surface temperatures and could comprise structural integrity of the system as well. The image sequence in Fig. 12 shows transition annular flow, intermittent CHF, rewetting and eventually the inverterd annular flow. Fig 12(a) shows rapid bubble nucleation near the entrance of the microchannel. The bubbles nucleate from the channel and depart downstream in the manifold region. However, no bubble


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Fig. 8. (a–e) Successive images of bubble mechanism in OMM geometry.

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Fig. 11. (a and b) Individual frames showing the annular flow and the transition flow regime. Fig. 9. Bubble nucleation, growth and departure mechanism at higher heat flux.

nucleation is seen from the downstream microchannel section, indicating dryout and presence of vapor in the channel. In the next sequence (Fig. 12(b)), due to the rapid nucleation at the microchannel entrance region, the rapid expansion and coalation of the bubbles leads to the onset of intermittent CHF. Fig 12(c) shows an intermittent CHF taking place. The entire microchannel surface is covered with vapor and no nucleation from within the channel region was seen. In the next image shown in Fig. 12(d), the oncoming liquid inertia overcomes the vapor blanket on the channel and bubble nucleation is observed at the inlet channel section. The downstream section however is still under CHF like condition. Rewetting and rapid nucleation resumes near the upstream section in the microchannel region (Fig. 12(e)). The image shows some rewetting in the downstream region, however the surface reverts to dryout conditions quickly. The image sequence seen

from Fig 12(a)–(e) is repeated multiple times, characterized by brief periods of dryout and then rewetting due to initiation of nucleation at the inlet. Eventually, the final image (Fig. 12(f)) showing that the CHF is reached with the inverted annular flow pattern. 3.7. Heat transfer mechanism for plain surface A plain surface under the tapered manifold geometry was also studied by Kalani and Kandlikar [28]. This configuration provides useful insight into the heat transfer mechanism. Fig. 13 shows a sequence of images of the bubble growth sequence for a plain surface with tapered manifold. Fig 13(a) displays multiple active nucleation sites with bubble nucleation and a larger departed bubble moving towards these sites from the left. Fig. 13(b) shows the bubble covering these nucleation sites and waves can be observed in the regions of the active nucleation sites. In the next sequence (Fig. 13(c)) the bubble has covered these nucleation sites. In the final image (Fig. 13(d)), dry spot formation is observed at the location of the nucleation site. However, no dry spots are observed on the remaining area occupied by the bubble, indicating presence of liquid film underneath the bubble. These dry spots are seen to be initiated from the nucleation sites covered by the oncoming large bubble. Nucleation is seen to resume once the bubble moved away from the site. This behavior is somewhat similar to a bubble growing in pool boiling, with nucleation, dryout and rewetting at the base of a bubble [29]. 3.8. Comparison of bubble nucleation mechanism between plain and microchannel surface

Fig. 10. Images showing bubble nucleation and growth in the channels under the vapor occupied manifold region at a higher heat flux.

For the plain surface, dryout is initiated when an oncoming departed bubble covers an active nucleation site as seen from the


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with liquid. When a departed bubble arrives over a nucleation site in the microchannel (Fig. 14), the bubble nucleation continues in the channel. The addition of microchannel and taper provides increased cross-sectional area which has significant impact on the bubble growth. This mechanism prevents dryout formation at low heat fluxes and provides a significant increase in heat transfer performance. 3.9. Comparison of closed microchannel and OMM geometry

Fig. 12. (a–f) Images showing the transition from annular to inverted annular flow.

Closed microchannels (microchannels with a cover plate) have been extensively studied by earlier researchers [5,16]. Balasubramanian and Kandlikar [30] reported flow instability due to explosive growth of bubbles in their flow boiling study as shown in Fig. 15. Rapid bubble growth in the microchannel causes the bubble to grow explosively in both upstream and downstream directions. The explosive bubble growth rate occurs due to the surrounding superheated liquid environment that the bubble experiences after nucleation. This rapid bubble growth leads to flow reversal and flow maldistribution in parallel channels. The walls of the microchannel remain exposed to the rapidly growing bubble, creating local dry patches and causing a reduction in the heat transfer performance. In certain cases, the bubble expands back into the inlet header due to flow reversal and causes severe pressure and temperature fluctuations. More importantly, these can lead to early CHF and damage the structural integrity of the system [22]. Inlet restrictors [8,31] have been used to overcome flow maldistribution, however the overall heat transfer performance with microchannels were still low. The bubble nucleation, growth and departure mechanism for the OMM geometry is seen in Fig. 16. The bubbles nucleating in the channels are termed as the primary bubbles, while the bubbles flowing in the manifold region are called secondary bubbles. As seen from Figs. 8–10, heat transfer performance is increased and flow instabilities are drastically reduced by providing space for secondary bubbles to expand and flow in the manifold region. The

Fig. 14. Schematic representation of the difference in the bubble nucleation mechanism for microchannel and plain surface.

Fig. 13. (a–d) Sequence of images showing the underlying mechanism for plain surface.

sequence of images shown in Fig 13. In case of a microchannel surface, nucleation occurs in the channel and the bubble grows and departs in the manifold region (Fig. 7), keeping the channel filled

Fig. 15. Closed microchannel showing flow instability (Balasubramanian and Kandlikar [25]).

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stream. Similarity between pool boiling and flow boiling in microchannels reported in Kandlikar [29] is also seen to be applicable to flow boiling in tapered OMM configuration. 9. Nucleation and bubble growth (primary bubble) in the microchannels and merger with the vapor in the manifold region (secondary bubble) are seen as the main mechanism responsible for enhanced heat transfer performance. 10. Unlike closed microchannels, which suffer from lateral expansion of bubble and low heat transfer performance, the OMM geometry is able to maintain stable flow and provide liquid supply to the channels and reach a very high heat flux of 506 W/cm2 at a wall superheat of 26.2 °C without reaching CHF.

Conflict of interest Fig. 16. Open microchannel with tapered manifold showing bubble dynamics.

None declared. localized expansion of a primary bubble in both upstream and downstream directions within the channel region does not have any detrimental effect on the overall stability of the system because of the large interconnected vapor space available in the manifold. The flow remains stable in the downstream direction. This resulted in a maximum heat flux of 506 W/cm2 was dissipated at a wall superheat of 26.2 °C with the OMM geometry [15]. 4. Conclusions High speed visualization was employed to investigate the flow patterns and heat transfer mechanisms for open microchannel with tapered manifold geometry. The following conclusions are made through this study: 1. Five flow patterns namely, bubbly flow, slug flow, intermittent slug/bubbly flow, annular flow and inverted annular flow (post-CHF) were identified with the OMM geometry. 2. The flow patterns were depicted on a flow pattern map using superficial gas and liquid velocities similar to Taitel–Dukler map. Significant deviations are noted between the transition boundaries for the OMM geometry as compared to the microchannels and in macroscale geometries. 3. For bubble nucleation process in the OMM geometry, the bubbles nucleated from the base of the channel and grew over the channel fin in the manifold region. 4. The departed bubbles occupied the space in the manifold region and expanded in the flow direction due to the taper. 5. The channels remain filled with liquid while the large bubbles flow in the manifold over the microchannels. Bubble nucleation continues to occur in the channel even when the vapor occupies the manifold area over it and similar mechanistic pattern is observed at higher heat fluxes. 6. Taper helps in reducing the pressure drop due to increasing flow area in the flow direction as postulated by Kalani and Kandlikar [26]. The taper also helps in improving the overall stability. The flow is not affected by the perturbations due to the nucleation and localized explosive growth of bubbles in the microchannel. 7. Localized lateral expansion of the bubbles in the channels did not have any adverse effect on the heat transfer performance due to the large available vapor space (manifold region) above the microchannels. 8. For the plain surface, dry spots were observed at active nucleation sites covered by an oncoming vapor bubble. This dry spot region grew until rewetted by the liquid

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