Subcooled flow boiling and microbubble emission boiling phenomena in a partially heated microchannel

A simultaneous visualization and measurement study has been carried out to investigate subcooled flow boiling and microbubble emission boiling (MEB) phenomena of deionized water in a partially heated Pyrex glass microchannel, having a hydraulic diameter of 155 μm, which was integrated with a Platinum microheater. Effects of mass flux, inlet water subcooling and surface condition of the microheater on subcooled flow boiling in microchannels are investigated. It is found that MEB occurred at high inlet subcoolings and at high heat fluxes, where vapor bubbles collapsed into microbubbles after contacting with the surrounding highly subcooled liquid. In the fully-developed MEB regime where the entire microheater was covered by MEB, the mass flux, the inlet water subcooling and the heater surface condition have only small effects on the boiling curves. The occurrence of MEB in microchannel can remove a large amount of heat flux, as high as 14.41 MW/m² at a mass flux of 883.8 kg/m² s, with only a moderate rise in wall temperature. Therefore, MEB is a very promising method for cooling of microelectronic chips. Heat transfer in the fully-developed MEB in the microchannel is presented, which is compared with existing subcooled flow boiling heat transfer correlations for macrochannels.     

Visualization of convective boiling heat transfer in single microchannels with different shaped cross-sections

Convective boiling in transparent single microchannels with similar hydraulic diameters but different shaped cross-sections was visualized, along with simultaneous measurement of the local heat transfer coefficient. Two types of microchannels were tested: a circular Pyrex glass microtube (210 μm inner diameter) and a square Pyrex glass microchannel (214 μm hydraulic diameter). A 100-nm-thick semi-transparent ITO/Ag thin film sputtered on the outer wall of the microchannel was used for direct joule heating of the microchannel.The flow field visualization showed semi-periodic variation in the flow patterns in both the square and circular microchannels. Such variation was because the confined space limited the bubble growth in the radial direction.In the square microchannel, both the number of nucleation bubbles and the local heat transfer coefficient increased with decreasing vapor quality. The corners acted as active nucleation cavities, leading to the higher local heat transfer coefficient. In contrast, lack of cavities in the smooth glass circular microchannel yielded a relatively smaller heat transfer coefficient at lower vapor quality. Finally, the heat transfer coefficient was higher for the square microchannel because corners in the square microchannel acted as effective active nucleation sites.     

Unstable and stable flow boiling in parallel microchannels and in a single microchannel

A simultaneous visualization and measurement study have been carried out to investigate flow boiling instabilities of water in microchannels at various heat fluxes and mass fluxes. Two separate flow boiling experiments were conducted in eight parallel silicon microchannels (with flow interaction from neighboring channels at headers) and in a single microchannel (without flow interaction), respectively. These microchannels, at a length of 30 mm, had an identical trapezoidal cross-section with a hydraulic diameter of 186 μm. At a given heat flux and inlet water temperature, it was found that stable and unstable flow boiling regimes existed, depending on the mass flux. A flow boiling map, in terms of heat flux vs mass flux, showing stable flow boiling regime and unstable flow boiling regime is presented for parallel microchannels as well as for a single microchannel, respectively, at an inlet water temperature of 35 °C. In the stable flow boiling regime, isolated bubbles were generated and were pushed away by the incoming subcooled liquid. Two unstable flow boiling regimes, with long-period oscillation (more than 1 s) and short-period oscillation (less than 0.1 s) in temperature and pressure, were identified. The former was due to the expansion of vapor bubble from downstream while the latter was owing to the flow pattern transition from annular to mist flow. A comparison of results of flow boiling in parallel microchannels and in a single microchannel shows that flow interaction effects from neighboring channels at the headers are significant.     

Nucleate and film boiling on a microheater under pulse heating in a microchannel

Using MEMS technology, a Pt microheater (60 × 100 µm²) fabricated on a glass wafer is placed in a silicon-based microchannel of trapezoidal cross section. With the aid of a high-speed CCD and based on Pt's linear temperature-resistance characteristic, flow boiling phenomena and temperature response on the surface of the microheater in the microchannel under pulse heating are observed and recorded. At a given mass flux, nucleate boiling and film boiling begin to appear on the microheater with increasing heat flux. A flow boiling map, showing the effects of heat and mass flux on nucleate and film boiling regimes on the microheater at a pulse heating width of 2 ms, is presented. It is found that nucleate boiling is changed to film boiling as the heat flux supplied to the microheater is increased. Furthermore, increasing mass flux increases the heat flux required for the incipience of nucleate boiling and film boiling on the microheater in the microchannel.     

Effects of pulse width and mass flux on microscale flow boiling under pulse heating

A Pt microheater (140 × 100 μm²) is fabricated on a glass wafer and enclosed in a silicon microchannel of trapezoidal cross section by MEMS technology. With the aid of a high-speed CCD and data acquisition system, subcooled flow boiling phenomena and temperature response on the surface of the microheater under pulse heating are observed and recorded. Experiments are conducted for six pulse widths (50 μs, 100 μs, 200 μs, 600 μs, 1 ms, and 2 ms) under different mass and heat fluxes. With increasing heat flux at a fixed pulse width and different mass fluxes, four flow regimes including single phase, nucleate boiling, film boiling and dry out are identified. Since flow boiling regimes are relatively independent of mass flux, correlation equations based on experimental data for the transitional heat flux of different flow boiling regimes are obtained in terms of pulse width only. It is also found that pulse width and mass flux have little influence on boiling inception time, and the classical analytical solution for the nucleation inception time in terms of heat flux is verified experimentally.     

Flow pattern transition instability during flow boiling in a single microchannel

We studied the unique characteristics of flow boiling in a single microchannel, including the periodic pressure drop, mass flow rate, and temperature fluctuations, in terms of a long time period. Experiments were conducted using a single horizontal microchannel and deionized water to study boiling instabilities at very small mass and heat flow rate conditions. A Polydimethylsiloxane (PDMS) rectangular single microchannel had a hydraulic diameter of 103.5 μm and a length of 40 mm. A series of piecewise serpentine platinum microheaters were fabricated on the inner bottom wall of the rectangular microchannel to supply thermal energy to the test fluid. Real-time flow visualizations of the flow pattern inside the microchannel were performed simultaneously with measurements of the experimental parameters. Tests were performed for mass fluxes of 170 and 360 kg/m² s and heat fluxes of 200–530 kW/m². The test results showed that the heated wall temperature, pressure drop, and mass flux all fluctuated with a long period and large amplitude. These periodic fluctuations exactly matched the transition of two alternating flow patterns inside the microchannel: a bubbly/slug flow and an elongated slug/semi-annular flow. Therefore, the flow pattern transition instability in the single microchannel caused a cyclic behavior of the wall temperature, pressure drop, and mass flux, and this behavior had a very long period (100–200 s) and large amplitude.     

Boiling heat transfer and critical heat flux of ethanol–water mixtures flowing through a diverging microchannel with artificial cavities

This paper presents an experimental study on the convective boiling heat transfer and the critical heat flux (CHF) of ethanol–water mixtures in a diverging microchannel with artificial cavities. The results show that the boiling heat transfer and the CHF are significantly influenced by the molar fraction (x_m) as well as the mass flux. For the single-phase convection region except for the region near the onset of nucleate boiling with temperature overshoot, the single-phase heat transfer coefficient is independent of the wall superheat and increases with a decrease in the molar fraction. After boiling incipience, the two-phase heat transfer coefficient is much higher than that of single-phase convection. The two-phase heat transfer coefficient shows a maximum in the region of bubbly-elongated slug flow and deceases with a further increase in the wall superheat until approaching a condition of CHF, indicating that the heat transfer is mainly dominated by convective boiling. A flow-pattern-based empirical correlation for the two-phase heat transfer coefficient of the flow boiling of ethanol–water mixtures is developed. The overall mean absolute error of the proposed correlation is 15.5%, and more than 82.5% of the experimental data were predicted within a ±25% error band. The CHF increases from x_m = 0–0.1, and then decreases rapidly from x_m = 0.1–1 at a given mass flux of 175 kg/m² s. The maximum CHF is reached at x_m = 0.1 due to the Marangoni effect, indicating that small additions of ethanol into water could significantly increase the CHF. On the other hand, the CHF increases with increasing the mass flux at a given molar fraction of 0.1. Moreover, the experimental CHF results are compared with existing CHF correlations of flow boiling of the mixtures in a microchannel.     

Two-phase flow in high-heat-flux micro-channel heat sink for refrigeration cooling applications: Part II—heat transfer characteristics

This paper is the second of a two-part study concerning two-phase flow and heat transfer characteristics of R134a in a micro-channel heat sink incorporated as an evaporator in a refrigeration cycle. Boiling heat transfer coefficients were measured by controlling heat flux (q″ = 15.9 − 93.8 W/cm²) and vapor quality (x_e = 0.26 − 0.87) over a broad range of mass velocity. While prior studies point to either nucleate boiling or annular film evaporation (convective flow boiling) as dominant heat transfer mechanisms in small channels, the present study shows heat transfer is associated with different mechanisms for low, medium and high qualities. Nucleate boiling occurs only at low qualities (x_e < 0.05) corresponding to very low heat fluxes, and high fluxes produce medium quality (0.05 < x_e < 0.55) or high quality (x_e > 0.55) flows dominated by annular film evaporation. Because of the large differences in heat transfer mechanism between the three quality regions, better predictions are possible by dividing the quality range into smaller ranges corresponding to these flow transitions. A new heat transfer coefficient correlation is recommended which shows excellent predictions for both R134a and water.     

Pressure Drop,Boiling Heat Transfer and Flow Patterns during Flow Boiling in a Single Microchannel

The boiling heat transfer and two-phase pressure drop of water in a microscale channel were experimentally investigated. The tested horizontal rectangular microchannel had a hydraulic diameter of 100 μ m and length of 40 mm. A series of microheaters provided heat energy to the working fluid, which made it possible to control and measure the local thermal conditions in the direction of the flow. Both the microchannel and microheaters were fabricated using a micro-electro-mechanical systems (MEMS) technique. Flow patterns were obtained from real-time flow visualizations made during the flow boiling experiments. Tests were performed for mass fluxes of 90, 169, and 267 kg/m²s and heat fluxes from 200 to 500 kW/m². The effects of the mass flux and vapor quality on the local flow boiling heat transfer coefficient and two-phase frictional pressure gradient were studied. The evaluated experimental data were compared with existing correlations. The experimental heat transfer coefficients were nearly independent of the mass flux and vapor quality. Most of the existing correlations did not provide reliable heat transfer coefficient predictions for different vapor quality values, nor could they predict the two-phase frictional pressure gradient except under some limited conditions.     

Flow Boiling Heat Transfer of Refrigerant R-134a in Copper Microchannel Heat Sink

In this paper we present experimental data on heat transfer and pressure drop characteristics at flow boiling of refrigerant R-134a in a horizontal microchannel heat sink. The primary objective of this study was to experimentally establish how the local heat transfer coefficient and pressure drop correlate with the heat flux, mass flux, and vapor quality. The copper microchannel heat sink contains 21 microchannels with 335 × 930 μm² cross section. The microchannel plate and heating block were divided by the partition wall for the local heat flux measurements. Distribution of local heat transfer coefficients along the length and width of the microchannel plate was measured in the range of external heat fluxes from 50 to 500 kW/m²; the mass flux varied within 200–600 kg/m²-s, and pressure varied within 6–16 bar. The obvious impact of heat flux on the magnitude of heat transfer coefficient was observed. It showed that nucleate boiling is the dominant mechanism for heat transfer. A new model of flow boiling heat transfer, considering nucleate boiling suppression and liquid film evaporation, was proposed and verified experimentally in this paper.     

A four-zone model for saturated flow boiling in a microchannel of rectangular cross-section

A four-zone flow boiling model is presented to describe saturated flow boiling heat transfer mechanisms in a microchannel of rectangular cross-section. The boiling process in the microchannel is assumed to be a cyclic passage of four zones: (i) liquid-slug zone, (ii) elongated bubble zone, (iii) partially-dryout zone, and (iv) fully-dryout zone. The existence of the partially-dryout zone in this model is proposed to take into consideration of corner effects on boiling heat transfer in the microchannel. To verify this new model, an experimental study was carried out to investigate flow boiling heat transfer of water in a microchannel having a rectangular cross-section with a hydraulic diameter of 137 μm (202 μm in width and 104 μm in depth) with a length of 30 mm under three-side heating condition. The data for bubble nucleation frequency was correlated in terms of the Boiling number, which was used to determine the heat transfer coefficient. It is found that the present four-zone flow boiling model successfully predicts trends of boiling heat transfer data in a microchannel with a rectangular cross-section, having a sharp peak at low vapor quality depending on the mass flow rate. The predictions of flow boiling heat transfer coefficient in the microchannel are found in good agreement with experimental data with a MAE of 13.9%.     

Critical heat flux on flow boiling of methanol–water mixtures in a diverging microchannel with artificial cavities

This study constitutes an experimental investigation into the convective boiling heat transfer and critical heat flux (CHF) of methanol–water mixtures in a diverging microchannel with artificial cavities. Flow visualization shows that bubbles are generally nucleated at both the artificial cavities and side walls of the channel. This confirms the proper functioning of such artificial cavities. Consequently, the wall superheat of the onset nucleate boiling is significantly reduced. Experimental results show that the boiling heat transfer and CHF are significantly influenced by the molar fraction (x_m) as well as the mass flux. The CHF increases with an increase in mass flux at the same molar fraction. On the other hand, the CHF increases slightly from x_m = 0 to 0.3, and then decreases rapidly from x_m = 0.3 to 1 at the same mass flux. The maximum CHF is reached at x_m = 0.3, particularly for a mass flux of 175 kg/m² s, due to the Marangoni effect. Flow visualization confirms that the Marangoni effect helps a region with a liquid film breakup persist to a higher heat flux, and therefore a higher CHF. Moreover, a new empirical correlation involving the Marangoni effect for the CHF on the flow boiling of methanol–water mixtures is developed. The present correlation prediction shows excellent agreement with the experimental data, and further confirms that the present correlation may predict the Marangoni effect on the CHF for the convective boiling heat transfer of binary mixtures.     

Experimental investigation of condensation of hydrocarbon refrigerants (R600a) in a horizontal smooth tube

In this study, the experimental results of the condensing heat transfer coefficients of R600a, a hydrocarbon refrigerant, in a horizontal smooth copper tube with an inner diameter of 4 mm and outer diameter of 6 mm are presented at different vapor quality and different mass fluxes during condensation under annular flow conditions, by adjusting the desired vapor qualities at the test area. A specially-designed sight glass has been fitted to the inlet and outlet of the test tube to identify the flow type by naked eye after the inlet vapor quality of the refrigerant to be fed to the test area during the test is adjusted in the system. Thanks to a new method developed in the measuring system, the condensing heat transfer coefficients could be calculated by measuring the difference value (T_s − T_w) directly from the data collection unit. The experimental findings have shown that the condensing heat transfer coefficients drops down with reduction in vapor quality and the coefficient rises with the increase in the mass flux at constant vapor quality. A correlation has been developed from the data obtained. The condensing heat transfer coefficients obtained from the experimental study were seen to be consistent by ± 20% with the correlations developed by Shah, Travis and Cavallini–Zecchin.     

Heat transfer characteristics of flow boiling in a single horizontal microchannel

Heat transfer characteristics of subcooled flow boiling of FC-72 in a single horizontal circular cross-section microchannel (480 μm i.d., 800 μm o.d., 102 mm long) are presented. Different flow patterns, both in the stable and unstable flow boiling regimes, have been captured using high speed video camera. Data in small, medium, high and very high heat flux cases under small, medium and high mass flux has been presented. Convective heat transfer coefficients in each flow boiling situation have been calculated and presented. Stable flow boiling with alternating bubbly/slug flow, slug/annular flow and annular/mist flow have been observed for heat flux of 150 kW/m² or higher and mass flux of 1500 kg/m² s or higher. Back and forth oscillations with flow instabilities have been observed in cases of lower heat and mass fluxes. However, no complete reverse flow in upstream direction has been observed.     

Condensation of steam in silicon microchannels

A visualization study was performed on condensation of steam in microchannels etched in a 〈100〉 silicon wafer that was bonded by a thin Pyrex glass plate from the top. The microchannels had a trapezoidal cross section with a hydraulic diameter of 75 μm. Saturated steam flowed through these parallel microchannels, whose walls were cooled by natural convection of air at room temperature. The absolute pressure of saturated steam at the inlet ranged from 127.5 kPa to 225.5 kPa, and the outlet was at atmospheric pressure at approximately 101.3 kPa with the outlet temperature of the condensate ranging from 42.8 °C to 90 °C. Stable droplet condensation was observed near the inlet of the microchannel. When the condensation process progressed along the microchannels, droplets accumulated on the wall. As the vapor core entrained and pushed the droplets, it became an intermittent flow of vapor and condensate at downstream of the microchannels. The traditional annual flow, wavy flow and dispersed flow observed during condensation in macrochannels were not observed in the microchannels. Based on a modified classical droplet condensation theory, it is predicted that the droplet condensation heat flux increases as the diameter of the microchannel is decreased. It is also predicted that the droplet condensation heat flux of saturated steam at 225.5 kPa can reach as high as 1200 W/cm² at ΔT=10 °C in a microchannel having a hydraulic diameter of 75 μm.     

Boiling of water and FC-72 in microchannels enhanced with novel features

A microchannel test section comprised of parallel square microchannels with a 25 × 25 μm and 50 × 50 μm cross section was manufactured. Boiling of perfluorinated dielectric fluid FC-72 and water in microchannels was studied. Troublesome occurrences associated with flow boiling in microchannels were reduced or eliminated with inlet/outlet restrictors, inlet/outlet manifolds and potential nucleation cavities incorporated in the array of microchannels. The gradual reduction of channel cross section in the manifolds ensured a uniform distribution of the working fluid among the microchannels. The flow restrictors provided a higher upstream pressure drop in comparison with the downstream pressure drop which favors vapor flow in the downstream direction and consequentially suppresses the vapor backflow present in flow boiling. The superheat of the microchannel wall necessary for the onset of boiling was decreased significantly with the incorporation of properly sized artificial cavities. Experimental results confirmed the benefits of the etched features, as there was (i) an even working fluid distribution (ii) without dominating backflows of vapor (iii) at a low temperature of the onset of boiling. Bubble growths as well as other events in the microchannels were visualized with a high-speed imaging system which captured images at over 87,000 frames per second. Results exhibit boiling hysteresis dependence of the working fluid and its mass flux through the microchannels. The temperature of the onset of boiling is highly dependent on the working fluid, microchannel size and its roughness.     

Fundamental Issues Related to Flow Boiling and Two-Phase Flow Patterns in Microchannels – Experimental Challenges and Opportunities

ABSTRACT

Flow boiling heat transfer in microchannels is used today in many diverse applications. The previous studies addressing the effect of channel size, heat flux, vapor quality, and mass flux on heat transfer during flow boiling are reviewed in the present paper. The relationship between flow characteristics and flow boiling heat transfer was studied experimentally for refrigerant R-C318 at moderate reduced pressures where the contribution of nucleate boiling is decisive. Flow boiling mechanisms were identified using an annular microchannel with transparent outer wall for successive visualization of boiling. The considerable suppression of nucleate boiling heat transfer was observed at transition to annular flow and explained by formation of a liquid flow with thin film and dry spots. A general equation for prediction of two-phase flow boiling heat transfer inside the circular, annular, and rectangular microchannels is proposed and verified using the experimental data. This equation accounts for the nucleate boiling suppression, forced convection, and thin film evaporative heat transfer in the form that allows to distinguish more clearly the contribution of each mechanism of heat transfer under the conditions, when it is predominant. A new approach for prediction of transition to the annular flow is proposed and verified, using the experimental data.     

Condensation flow patterns in silicon microchannels

A simultaneous visualization and measurement experiment was carried out to investigate condensation flow patterns of steam flowing through an array of trapezoidal silicon microchannels, having a hydraulic diameter of 82.8 μm and a length of 30 mm. The degassed and deionized water steam flowing in the microchannels was cooled by flowing water of 8 °C from the bottom. The silicon microchannels were covered with a thin transparent pyrex glass from the top which enabled the visualization of flow patterns. Experiments were performed at different inlet pressures ranging from 4.15 × 10⁵ Pa to 1.25 × 10⁵ Pa (with corresponding mass fluxes decreasing from 47.5 g/cm² s to 19.3 g/cm² s) while the outlet pressure was maintained at a value of 10⁵ Pa. Different condensation flow patterns such as fully droplet flow, droplet/annular/injection/slug-bubbly flow, annular/injection/slug-bubbly flow, and fully slug-bubbly flow were observed in the microchannels. At a given inlet pressure and mass flux, the flow pattern depended on both the location and time. Of particular interest is that the vapor injection flow, consisting of a series of bubble growth and detachment activities, appeared and disappeared periodically. During the disappearance period of injection flow, the slug-bubbly flow at downstream changed to the single-phase liquid flow due to the reversed flow of outlet condensate, while the annular flow at upstream changed to the vapor flow due to the effect of incoming vapor. Therefore, two-phase flow and single-phase flow appeared alternatively in the microchannels, causing large fluctuations of wall temperatures as well as other measurements. It was also found that the occurrence of vapor injection flow moved from the outlet toward the inlet as the mass flux was decreased. The vapor injection flow and its induced condensation instabilities in microchannels are reported here for the first time.     

Microchannel size effects on local flow boiling heat transfer to a dielectric fluid

Heat transfer with liquid–vapor phase change in microchannels can support very high heat fluxes for use in applications such as the thermal management of high-performance electronics. However, the effects of channel cross-sectional dimensions on the two-phase heat transfer coefficient and pressure drop have not been investigated extensively. In the present work, experiments are conducted to investigate the local flow boiling heat transfer of a dielectric fluid, Fluorinert FC-77, in microchannel heat sinks. Experiments are performed for mass fluxes ranging from 250 to 1600 kg/m² s. Seven different test pieces made from silicon and consisting of parallel microchannels with nominal widths ranging from 100 to 5850 μm, all with a nominal depth of 400 μm, are considered. An array of temperature sensors on the substrate allows for resolution of local temperatures and heat transfer coefficients. The results of this study show that for microchannels of width 400 μm and greater, the heat transfer coefficients corresponding to a fixed wall heat flux as well as the boiling curves are independent of channel size. Also, heat transfer coefficients and boiling curves are independent of mass flux in the nucleate boiling region for a fixed channel size, but are affected by mass flux as convective boiling dominates. A strong dependence of pressure drop on both channel size and mass flux is observed. The experimental results are compared to predictions from a number of existing correlations for both pool boiling and flow boiling heat transfer.     

Study of the critical heat flux condition with water and R-123 during flow boiling in microtubes. Part I: Experimental results and discussion of parametric effects

Extensive experimentation was performed to obtain flow boiling critical heat flux data in single stainless steel microtubes with diameters from 0.286 to 0.700 mm over a wide range of mass fluxes, inlet subcoolings, and exit pressures for two different working fluids (water and R-123). The effect of different operating parameters – mass flux, inlet subcooling, exit quality, heated length and diameter – were assessed in detail (Part I of the paper). The conventional DNB-type behavior is observed in the high subcooled region, and the typical dryout type behavior is seen in the high-quality saturated region when the flow is completely annular. The flow in transitional flow patterns (churn–annular or slug–annular) causes a peculiar increase of CHF with exit quality. Also, the increased void fraction near the saturated region in subcooled boiling results in increased subcooled CHF values. Part II of the paper deals with comparison of data with existing correlations and development of a new correlation to predict the CHF condition in the subcooled liquid region.