Saturated flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip

An experiment is carried out here to investigate flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip flush-mounted in the bottom of a horizontal rectangular channel. Besides, three different micro-structures of the chip surface are examined, namely, the smooth, pin-finned 200 and pin-finned 100 surfaces. The pin-finned 200 and 100 surfaces, respectively, contain micro-pin-fins of size 200 μm × 200 μm × 70 μm (width × length × height) and 100 μm × 100 μm × 70 μm. The pitch of the fins is equal to the fin width for both surfaces. The effects of the FC-72 mass flux, imposed heat flux, and surface micro-structures of the silicon chip on the FC-72 saturated flow boiling characteristics are examined in detail. The experimental data show that an increase in the FC-72 mass flux causes a delay in the boiling incipience. However, the flow boiling heat transfer coefficient is not affected by the coolant mass flux. But adding the micro-pin-fin structures to the chip surfaces can effectively enhance the single-phase convection and flow boiling heat transfer. Moreover, the mean bubble departure diameter and active nucleation site density are reduced for a rise in the FC-72 mass flux. A higher coolant mass flux results in a higher mean bubble departure frequency. Furthermore, larger bubble departure diameter, higher bubble departure frequency, and higher active nucleation site density are observed at a higher imposed heat flux. We also note that adding the micro-pin-fins to the chips decrease the bubble departure diameter and increase the bubble departure frequency. However, the departing bubbles are larger for the pin-finned 100 surface than the pin-finned 200 surface but the bubble departure frequency exhibits an opposite trend. Finally, empirical equations to correlate the present data for the FC-72 single-phase liquid convection and saturated flow boiling heat transfer coefficients and for the bubble characteristics are provided.     

Correlation of critical heat flux and two-phase friction factor for subcooled convective boiling in structured surface microchannels

Critical heat flux (CHF) and pressure drop of subcooled flow boiling are measured for a microchannel heat sink containing 75 parallel 100 μm × 200 μm structured surface channels. The heated surface is made of a Cu metal sheet with/without 2 μm thickness diamond film. Tests and measurements are conducted with de-ionized water, de-ionized water +1 vol.% MCNT additive solution, and FC-72 fluids over a mass velocity range of 820–1600 kg/m² s, with inlet temperatures of 15(8.6)°C, 25(13.6)°C, 44(24.6)°C, and 64(36.6)°C for DI water (FC-72), and heat fluxes up to 600 W/cm². The CHF of subcooled flow boiling of the test fluids in the microchannels is measured parametrically. The two-phase pressure drop is also measured. Both CHF and the two-phase friction factor correlation for one-side heating with two other side-structured surface microchannels are proposed and developed in terms of the relevant parameters.     

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.     

Experimental study of saturated flow boiling heat transfer in an array of staggered micro-pin-fins

This study concerns water saturated flow boiling heat transfer in an array of staggered square micro-pin-fins having a 200 × 200 μm² cross-section by a 670 μm height. Three inlet temperatures of 90, 60, and 30 °C, six mass velocities for each inlet temperature, ranging from 183 to 420 kg/m² s, and outlet pressures between 1.03 and 1.08 bar were tested. Heat fluxes ranged from 23.7 to 248.5 W/cm². Heat transfer coefficient was fairly constant at high quality, insensitive to both quality and mass velocity. Heat transfer was enhanced by inlet subcooling at low quality. Possible heat transfer mechanism was discussed.     

Heat transfer characteristics during subcooled flow boiling of a kerosene kind hydrocarbon fuel in a 1 mm diameter channel

The subcooled flow boiling heat transfer characteristics of a kerosene kind hydrocarbon fuel were investigated in an electrically heated horizontal tube with an inner diameter of 1.0 mm, in the range of heat flux: 20–1500 kW/m², fluid temperature: 25–400 °C, mass flux: 1260–2160 kg/m² s, and pressure: 0.25–2.5 MPa. It was proposed that nucleate boiling heat transfer mechanism is dominant, as the heat transfer performance is dependent on heat flux imposed on the channel, rather than the fuel flow rate. It was found that the wall temperatures along the test section kept constant during the fully developed subcooled boiling (FDSB) of the non-azeotropic hydrocarbon fuel. After the onset of nucleate boiling, the temperature differences between inner wall and bulk fluid begin to decrease with the increase of heat flux. Experimental results show that the complicated boiling heat transfer behavior of hydrocarbon fuel is profoundly affected by the pressure and heat flux, especially by fuel subcooling. A correlation of heat transfer coefficients varying with heat fluxes and fuel subcooling was curve fitted. Excellent agreement is obtained between the predicted values and the experimental data.     

On the rise paths of single vapor bubbles after the departure from nucleation sites in subcooled upflow boiling

A photographic study was carried out for the subcooled flow boiling of water to elucidate the rise characteristics of single vapor bubbles after the departure from nucleation sites. The test section was a transparent glass tube of 20 mm in inside diameter and the flow direction was vertical upward; liquid subcooling was parametrically changed within 0–16 K keeping system pressure and liquid velocity at 120 kPa and 1 m/s, respectively. The bubble rise paths were analyzed from the video images that were obtained at the heat flux slightly higher than the minimum heat flux for the onset of nucleate boiling. In the present experiments, all the bubbles departed from their nucleation sites immediately after the inception. In low subcooling experiments, bubbles slid upward and consequently were not detached from the vertical heated wall; the bubble size was increased monotonously with time in this case. In moderate and high subcooling experiments, bubbles were detached from the wall after sliding for several millimeters and migrated towards the subcooled bulk liquid. The bubbles then reversed the direction of lateral migration and were reattached to the wall at moderate subcooling while they collapsed due to the condensation at high subcooling. It was hence considered that the mechanisms of the heat transfer from heated wall and the axial growth of vapor volume were influenced by the difference in bubble rise path. It was observed after the inception that bubbles were varied from flattened to more rounded shape. This observation suggested that the bubble detachment is mainly caused by the change in bubble shape due to the surface tension force.     

Subcooled boiling incipience on a highly smooth microheater

Subcooled boiling incipience on a highly smooth microscale heater (270 μm × 270 μm) submerged in FC-72 liquid is investigated. Using high-speed imaging and a transient heat flux measurement technique, the mechanics of homogeneous nucleation on the heater are elucidated. Bubble incipience on the microheater was observed to be an explosive process. It is found that the superheat limit of boiling liquid is required for bubble incipience. It is concluded that boiling incipience on the microheater is a homogeneous liquid–vapor phase change process. This is in contrast to recent observations of low-superheat heterogeneous nucleation on metallic surfaces of rms roughness ranging from 4 to 28 nm [T.G. Theofanous, J.P. Tu, A.T. Dinh, T.N. Dinh, The boiling crisis phenomenon part I: nucleation and nucleate boiling heat transfer, Exp. Therm. Fluid Sci. 26 (2002) 775–792; Y. Qi, J.F. Klausner, Comparison of gas nucleation and pool boiling site densities, J. Heat Transfer 128 (2005) 13–20; Y. Qi, J.F. Klausner, Heterogeneous nucleation with artificial cavities, J. Heat Transfer 127 (2005) 1189–1196]. Following the explosive bubble incipience, the boiling process on the microheater can be maintained at much lower superheats. This is mainly due to the necking during bubble departure that leaves an embryo from which the next-generation bubbles grow.     

Effects of high subcooling on two-phase spray cooling and critical heat flux

Experiments were performed with FC-77 using three full-cone spray nozzles to assess the influence of subcooling on spray performance and critical heat flux (CHF) from a 1.0 × 1.0 cm² test surface. The relatively high boiling point of FC-77 (97 °C at one atmosphere) enabled testing at relatively high levels of subcooling. Increasing the subcooling delayed the onset of boiling but decreased the slope of the nucleate boiling region of the spray boiling curve. The enhancement in CHF was relatively mild at low subcooling and more appreciable at high subcooling. CHF was enhanced by about a 100% when subcooling was increased from 22 to 70 °C, reaching values as high as 349 W/cm². The FC-77 data were combined with prior spray CHF data from several studies into a broad CHF database encompassing different nozzles, fluids, flow rates, spray orientations, and subcoolings. The entire CHF database was used to modify the effect of subcooling in a previous CHF correlation that was developed for relatively low subcoolings. The modified correlation shows excellent predictive capability.     

Wall heat flux partitioning during subcooled forced flow film boiling of water on a vertical surface

Subcooled flow film boiling experiments were conducted on a vertical flat plate, 30.5 cm in height, and 3.175 cm wide with forced convective upflow of subcooled water at atmospheric pressure. Data have been obtained for mass fluxes ranging from 0 to 700 kg/m²s, inlet subcoolings ranging from 0 to 25 °C and wall superheats ranging from 200 to 400 °C. Correlations for wall heat transfer coefficient and wall heat flux partitioning were developed as part of this work. These correlations derive their support from simultaneous measurements of the wall heat flux, fluid temperature profiles, liquid side heat flux and interfacial wave behavior during steady state flow film boiling. A new correlation for the film collapse temperature was also deduced by considering the limiting case of heat flux to the subcooled liquid being equal to the wall heat flux. The premise of this deduction is that film collapse under subcooled conditions occurs when there is no net vapor generation. These correlations have also been compared with the data and correlations available in the literature.     

Subcooled flow boiling heat transfer and associated bubble characteristics of R-134a in a narrow annular duct

Experiments are conducted here to investigate how the channel size affects the subcooled flow boiling heat transfer and associated bubble characteristics of refrigerant R-134a in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm in this study. From the measured boiling curves, the temperature undershoot at ONB is found to be relatively significant for the subcooled flow boiling of R-134a in the duct. The R-134a subcooled flow boiling heat transfer coefficient increases with a reduction in the gap size, but decreases with an increase in the inlet liquid subcooling. Besides, raising the imposed heat flux can cause a substantial increase in the subcooled boiling heat transfer coefficient. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are small in the narrow duct. Visualization of the subcooled flow boiling processes reveals that the bubbles are suppressed to become smaller and less dense by raising the refrigerant mass flux and inlet subcooling. Moreover, raising the imposed heat flux significantly increases the bubble population, coalescence and departure frequency. The increase in the bubble departure frequency by reducing the duct size is due to the rising wall shear stress of the liquid flow, and at a high imposed heat flux many bubbles generated from the cavities on the heating surface tend to merge together to form big bubbles. Correlation for the present subcooled flow boiling heat transfer data of R-134a in the narrow annular duct is proposed. Additionally, the present data for some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density are also correlated.     

Effects of water subcooling on heat transfer in vertical annuli

To obtain effects of major geometric parameters on subcooled heat transfer in vertical annuli three gap sizes (7.05, 18.15, and 28.2 mm) and two bottom conditions (open or closed) have been investigated experimentally in water at atmospheric pressure. Up to 50 °C of liquid subcooling has been tested and 429 data points were obtained at both single phase and boiling regions. The increase in pool subcooling results in much change in heat transfer coefficients. The governing mechanisms are suggested as single-phase natural convection and liquid agitation for the annuli with open bottoms while liquid agitation and bubble coalescence are the major factors at the bottom-closed annuli. Four empirical correlations for the heat transfer coefficient have been suggested in terms of the gap size, the degree of subcooling, and the heat flux. The correlations predict the heat transfer data of single phase and of boiling within ±5% and ±20%, respectively.     

Subcooled flow boiling heat transfer of R-407C and associated bubble characteristics in a narrow annular duct

An experiment is conducted here to investigate how the channel size affects the subcooled flow boiling heat transfer and the associated bubble characteristics of refrigerant R-407C in a horizontal narrow annular duct with the gap of the duct fixed at 1.0 and 2.0 mm. The measured boiling curves indicate that the temperature overshoot at ONB is relatively significant for the subcooled flow boiling of R-407C in the duct. Besides, the subcooled flow boiling heat transfer coefficient increases with a reduction in the duct gap, but decreases with an increase in the inlet liquid subcooling. Moreover, raising the heat flux imposed on the duct can cause a significant increase in the boiling heat transfer coefficients. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are slighter. Visualization of the subcooled flow boiling processes in the duct reveals that the bubbles are suppressed to become smaller and less dense by raising the refrigerant mass flux and inlet subcooling. Raising the imposed heat flux, however, produces positive effects on the bubble population, coalescence and departure frequency. Meanwhile, the present heat transfer data for R-407C are compared with the R-134a data measured in the same duct and with some existing correlations. We also propose empirical correlations for the present data for the R-407C subcooled flow boiling heat transfer and some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density.     

Effects of heater size and working fluids on nucleate boiling heat transfer

The present study is an experimental investigation of nucleate boiling heat transfer mechanism in pool boiling from wire heaters immersed in saturated FC-72 coolant and water. The vapor volume flow rate departing from a wire during nucleate boiling was determined by measuring the volume of bubbles from the wire utilizing the consecutive-photo method. The effects of the wire size on heat transfer mechanism during a nucleate boiling were investigated, varying 25 μm, 75 μm, and 390 μm, by measuring vapor volume flow rate and the frequency of bubbles departing from a wire immersed in saturated FC-72. One wire diameter of 390 μm was selected and tested in saturated water to investigate the fluid effect on the nucleate boiling heat transfer mechanism. Results of the study showed that an increase in nucleate boiling heat transfer coefficients with reductions in wire diameter was related to the decreased latent heat contribution. The latent heat contribution of boiling heat transfer for the water test was found to be higher than that of FC-72. The frequency of departing bubbles was correlated as a function of bubble diameters.     

Pool boiling of perfluorocarbon mixtures on silicon surfaces

The need for higher pool boiling critical heat flux (CHF) in electronic cooling applications has turned attention to the use of binary mixtures of dielectric liquids. The available literature demonstrates that the addition of a liquid with higher saturation temperature, higher molecular weight, higher viscosity and higher surface tension can lead to significant enhancement of CHF, beyond what can be achieved through changes in pressure, liquid subcooling, and the product of surface effusivity and heater thickness. The current study focuses on extending the available data on mixture CHF enhancement, as well as pool boiling, on polished silicon surfaces to FC-72/FC-40 mixture ratios of 10%, 15%, and 20% of FC-40 by weight, a pressure range of between 1 and 3 atm, and fluid temperature from 22 to 45 °C, leading to high subcooling conditions. It is found that peak heat flux can be increased to as high as 56.8 W/cm² compared to 25.2 W/cm² for pure FC-72 at 3 atm and 22 °C. It is believed that the increase in the mixture latent heat of evaporation and surface tension, accompanying the depletion of the lower boiling point fluid in the wall region plays the major role in enhancing the critical heat flux for binary mixtures.     

Local measurement of flow boiling in structured surface microchannels

Experiments were conducted to investigate flow boiling in 200 μm × 253 μm parallel microchannels with structured reentrant cavities. Flow morphologies, boiling inceptions, heat transfer coefficients, and critical heat fluxes were obtained and studied for mass velocities ranging from G = 83 kg/m² s to G = 303 kg/m² s and heat fluxes up to 643 W/cm². Comparisons of the performance of the enhanced and plain-wall microchannels were performed. The microchannels with reentrant cavities were shown to promote nucleation of bubbles and to support significantly better reproducibility and uniformity of bubble generation. The structured surface was also shown to significantly reduce the boiling inception and to enhance the critical heat flux.     

Single-phase and two-phase cooling with an array of rectangular jets

Experiments were performed to explore the effects of jet width, impingement velocity, and inlet subcooling on the cooling performance of an array of three confined rectangular FC-72 and ethanol jets impacting a 3.0 cm × 3.0 cm heated surface. The single-phase heat transfer coefficient increased with increasing jet velocity and/or jet width. A correlation for single-phase cooling was constructed by dividing the flow into impingement zones and confinement channel flow regions that are dominated by wall jet flow. Increases in jet velocity, jet width, and/or subcooling broadened the single-phase region preceding the commencement of boiling and enhanced critical heat flux (CHF). A new correlation was developed which fits the CHF data with good accuracy. Overall, better cooling performance was realized for a given flow rate by decreasing jet width. Pressure drop was for the most part quite modest, even for the smallest jet width and highest velocity tested. Overall, these results prove the present cooling scheme is highly effective at maintaining fairly isothermal surface conditions, with spatial variations of less than 1.2 and 2.6 °C for the single-phase and boiling regions, respectively. These results demonstrate the effectiveness of the present jet-impingement scheme for thermal management of next generation electronics devices and systems.     

Experimental investigation and modelling of heat transfer during convective boiling in a minichannel

Flow boiling heat transfer characteristics of water are experimentally studied in a circular minichannel with an inner diameter of 1500 μm. The fluid flows upwards and the test section, made of the nickel alloy Inconel 600, is directly electrically heated. Thus, the evaporation takes place under the defined boundary condition of constant heat flux. Mass fluxes between 50 and 100 kg/(m² s) and heat fluxes from 10 to 115 kW/m² at an inlet pressure of 3 bar are examined.Infrared thermography is applied to measure the outer wall temperatures of the minichannel. This experimental method permits the identification of different boiling regions, boiling mechanisms and the determination of local heat transfer coefficients. Measurements are carried out in single-phase flow, subcooled and saturated boiling regions. The experimental heat transfer coefficients in the region of saturated boiling are compared with correlations available in literature and with a physically founded model developed for convective boiling.     

Enhanced flow boiling heat transfer of FC-72 on micro-pin-finned surfaces

For the purpose of cooling electronic components with high heat flux efficiently, some experiments were conducted to study the flow boiling heat transfer performance of FC-72 on silicon chips. Micro-pin-fins were fabricated on the chip surface using a dry etching technique to enhance boiling heat transfer. Three different fluid velocities (0.5, 1 and 2 m/s) and three different liquid subcoolings (15, 25 and 35 K) were performed, respectively. A smooth chip (chip S) and four micro-pin-finned chips with the same fin thickness of 30 μm and different fin heights of 60 μm (chip PF30–60) and 120 μm (chip PF30–120), respectively, were tested. All the micro-pin-finned surfaces show a considerable heat transfer enhancement compared to the smooth one, and the critical heat flux increases in the order of chip S, PF30–60 and PF30–120. For a lower ratio of fin height to fin pitch and/or higher fluid velocity, the fluid velocity has a positive effect on the nucleate boiling curves for the micro-pin-finned surfaces. At the velocities lower than 1 m/s, the micro-pin-finned surfaces show a sharp increase in heat flux with increasing wall superheat, and the wall temperature at the critical heat flux (CHF) is less than the upper limit, 85 °C, for the reliable operation of LSI chips. The CHF values for all surfaces increase with fluid velocity and subcooling. The maximum CHF can reach nearly 150 W/cm² for chip PF30–120 at the fluid velocity of 2 m/s and the liquid subcooling of 35 K.     

Local heat transfer distribution and effect of instabilities during flow boiling in a silicon microchannel heat sink

Flow boiling of the perfluorinated dielectric fluid FC-77 in a silicon microchannel heat sink is investigated. The heat sink contains 60 parallel microchannels each of 100 μm width and 389 μm depth. Twenty-five evenly distributed temperature sensors in the substrate yield local heat transfer coefficients. The pressure drop across the channels is also measured. Experiments are conducted at five flow rates through the heat sink in the range of 20–80 ml/min with the inlet subcooling held at 26 K in all the tests. At each flow rate, the uniform heat input to the substrate is increased in steps so that the fluid experiences flow regimes from single-phase liquid flow to the occurrence of critical heat flux (CHF). In the upstream region of the channels, the flow develops from single-phase liquid flow at low heat fluxes to pulsating two-phase flow at high heat fluxes during flow instability that commences at a threshold heat flux in the range of 30.5–62.3 W/cm² depending on the flow rate. In the downstream region, progressive flow patterns from bubbly flow, slug flow, elongated bubbles or annular flow, alternating wispy-annular and churn flow, and wall dryout at highest heat fluxes are observed. As a result, the heat transfer coefficients in the downstream region experience substantial variations over the entire heat flux range, based on which five distinct boiling regimes are identified. In contrast, the heat transfer coefficient midway along the channels remains relatively constant over the heat flux range tested. Due to changes in flow patterns during flow instability, the heat transfer is enhanced both in the downstream region (prior to extended wall dryout) and in the upstream region. A previous study by the authors found no effect of instabilities during flow boiling in a heat sink with larger microchannels (each 300 μm wide and 389 μm deep); it appears therefore that the effect of instabilities on heat transfer is amplified in smaller-sized channels. While CHF increases with increasing flow rate, the pressure drop across the channels has only a minimal dependence on flow rate once boiling is initiated in the microchannels, and varies almost linearly with increasing heat flux.     

Enhanced boiling of HFE-7100 dielectric liquid on porous graphite

Enhanced boiling of HFE-7100 dielectric liquid on porous graphite measuring 10 mm × 10 mm is investigated, and results are compared with those for smooth copper (Cu) of the same dimensions. Although liquid is out-gassed for hours before performing the pool boiling experiments, air entrapped in re-entrant type cavities, ranging in size from tens to hundreds of microns, not only enhanced the nucleate boiling heat transfer and the critical heat flux (CHF), but also, the mixing by the released tiny air bubbles from the porous graphite prior to boiling incipience enhanced the natural convection heat transfer by ∼19%. No temperature excursion is associated with the nucleate boiling on porous graphite, which ensues at very low surface superheat of 0.5–0.8 K. Conversely, the temperature overshoot at incipient boiling on Cu is as much as 39.2, 36.6, 34.1 and 32.8 K in 0 (saturation), 10, 20 and 30 K subcooled boiling, respectively. Nucleate boiling ensues on Cu at a surface superheat of 11.9, 10.9, 9.5 and 7.5 K in 0 (saturation), 10, 20 and 30 K subcooled boiling, respectively. The saturation nucleate boiling heat flux on porous graphite is 1700% higher than that on Cu at a surface superheat of ∼10 K and decreases exponentially with increased superheat to ∼60% higher near CHF. The CHF values of HFE-7100 on porous graphite of 31.8, 45.1, 55.9 and 66.4 W/cm² in 0 (saturation), 10, 20 and 30 K subcooled boiling, are 60% higher and the corresponding superheats are 25% lower than those on Cu. In addition, the rate of increase in CHF with increased liquid subcooling is 50% higher than that on Cu.