Effect of channel length on critical heat flux under conditions of high subcooling and high velocity in short heated channels

The characteristics of critical heat flux (CHF) in existing experiments under high subcooling and high velocity in short heated channels have, for the first time, been systematically and quantitatively investigated to provide a CHF correlation that can properly predict the effect of channel length, especially when the channel length-to-channel diameter ratio L/D is less than about 20. The major test conditions of existing CHF experiments investigated in this study were channel diameter 1 to 4 mm, L/D 1 to 25, 0.1 to 1.2 MPa pressure, 34 to 117°C inlet water subcooling and 500 to 40 700 kg/(m² · s) mass flux in circular channels, and 3 to 20 mm gap size, 6 to 40 L/D_e, 0.1 to 3.1 MPa pressure, 4 to 166°C inlet water subcooling, and 940 to 27,000 kg/(m² · s) mass flux in rectangular channels. The effect of L/D on CHF was evaluated referring to the analytical solution of CHF, which was previously derived by the author for the channel flow at high subcooling and high velocity. As a result, the effect of L/D was quantitatively clarified as an effect of magnitude in heat transfer of single-phase forced-convection flow, giving a larger CHF with a smaller L/D in the case of L/D less than about 20. The proposed correlation predicts CHF to within a ±35 percent error margin. ©1998 Scripta Technica, Heat Trans Jpn Res, 27(7): 509–521, 1998     

High Heat Flux Burnout in Subcooled Flow Boiling

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Forced convective boiling of subcooled water at high velocity

This paper deals with heat transfer and critical heat flux (CHF) in subcooled flow boiling offering a fundamental study aimed at high heat flux cooling. Experiments with water at 0.12 MPa were conducted in a mass velocity range from 500 kg/m²s to 15,000 kg/m²s (velocity from 0.5 m/s to 15 m/s) and subcooling from 20 K to 60 K. A sheet of stainless steel (80 mm in heated length, 10 mm wide, and 0.2 mm thick) was mounted flush with a sidewall of a vertical rectangular channel (cross-section 20 mm by 30 mm) and heated directly using direct current. It was found that mass velocity and subcooling strongly affect CHF and heat transfer in non-boiling convection and partial nucleate boiling regimes. These two parameters have no appreciable influence in the fully developed nucleate boiling regime. In the parameter range used, CHF reached 15 MW/m². Boiling bubble behavior just prior to reaching CHF was found to vary depending on mass velocity and subcooling. 1998 Scripta Technica, Heat Trans Jpn Res, 27(5): 376–389, 1998     

Critical Heat Flux of R134a and R245fa Inside Small-Diameter Tubes

This article presents new experimental critical heat flux results under saturated flow boiling conditions for a macro-/microscale tube. The data were obtained in a horizontal 2.20-mm inside diameter stainless-steel tube with heating lengths of 361 and 154 mm, R134a and R245fa as working fluids, mass velocities ranging from 100 to 1500 kg/m²-s, critical heat flux from 25 to 300 kW/m², exit saturation temperatures of 25, 31, and 35°C, and critical vapor qualities ranging from 0.55 to 1. The experimental results show that critical heat flux (CHF) increases with increasing mass velocity and inlet subcooling but decreases with increasing saturation temperature and heated length. The data also indicated a higher CHF for R245fa when compared with R134a at similar conditions. The experimental data were compared against four CHF predictive methods and the results of the comparisons are reported.     

Heat transfer for flow boiling of water and critical heat flux in a half‐heated round tube under low‐pressure conditions

Heat transfer for flow boiling of water and critical heat flux (CHF) experiments in a half‐circumferentially heated round tube under low‐pressure conditions were carried out. To clarify the flow patterns in the heated section, experiments in the round tube under the same conditions were also carried out, and their results were compared. The experiments were conducted with atmospheric‐pressure water in test sections with inner diameter D = 6 mm, heated length L = 360 mm, inlet water subcooling ΔT_in = 80 K, and mass velocity G from 0 to 2000 kg/(m²·s) for the half‐circumferentially heated round tube and from 0 to 7000 kg/(m²·s) for the full‐circumferentially heated tube. The experimental data demonstrated that the wall temperature near the outlet of the half‐circumferentially heated tube remained almost the same until CHF. It was found that burnout occurred when the flow regime changed from churn flow to annular flow, and the liquid film on the heated wall dried out although liquid film on the unheated wall remained. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(3): 149–164, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10022     

Critical Heat Flux During Flow Boiling in Microchannels: A Parametric Study

The application of flow boiling in microchannels in copper cooling elements for very high heat flux dissipation from microprocessor chips is one of the promising technologies to replace air cooling and water cooling of these units, particularly in mainframes and servers. Recently, the authors have proposed a new theoretical model to predict the critical heat flux (CHF) in microchannels, and it is used here to perform a parametric study to investigate the effects of fluid, saturation temperature, mass flux, inlet subcooling, microchannel diameter, and heated length on CHF for this application. The parametric study shows that CHF is increased by: (i) decreasing channel length, (ii) lowering saturation temperature, (iii) increasing mass flux, (iv) increasing inlet subcooling, and (v) increasing microchannel diameter. The best coolant is water, but water is not feasible for the present application because of its very low saturation pressure at 30–40°C. Of the other four fluids simulated, their order of merit from best to worst is as follows: R-245fa, R-134a, R-236fa, and FC-72. FC-72, however, has a low saturation pressure (in fact, it would operate under vacuum at the saturation temperatures of 30–40°C envisioned here) and is not a candidate fluid for the flow boiling coolant here. Furthermore, the authors have also recently proposed a diabatic flow map for microchannels based on their database for R-134a and R-245fa in 0.5- and 0.8-mm channels. The new CHF model has been incorporated into their map here to predict the transition from annular flow to dry-out, which is a critical design limitation for microprocessor coolers. Importantly, this map then provides the feasible operating range of such coolers with flow boiling as the cooling process, in terms of mass flux and maximum vapor quality at the outlet to avoid CHF.     

Critical heat flux in non‐uniformly heated tube under low‐pressure and low‐mass‐flux condition

In an actual boiling channel, e.g., a boiler water‐tube, the circumferential heat flux is not uniform. Thus, the critical heat flux (CHF) of a non‐uniformly heated tube becomes an important design factor for conventional boilers, especially for a compact water‐tube boiler with a tube‐nested combustor. A small compact boiler is operated under low‐pressure and low‐mass‐flux conditions compared with a large‐scale boiler, thus the redistribution of liquid film strongly affects the characteristics of CHF. In this investigation, non‐uniform heat flux distribution along the circumferential direction was generated by using the Joule heating of SUS304 tubes with the wall thickness distribution. The heated length of test‐section was 900 mm with an inner diameter of 20 mm and an outer diameter of 24 mm. The center of the inner tube surface was shifted by ε=0, 0.5, 1.0, 1.5 mm from the center of the outer tube surface. The heat flux ratio between maximum and minimum heat flux of these tubes corresponded to 1.0, 1.7, 3.0, and 7.0, respectively. The experimental conditions were as follows: system pressure at 0.3 and 0.4 MPa, mass flux of 10–100kg/(m²s), inlet temperatures at 30° and 80°. The experimental results showed an increase in the critical heat flux substantiated by the existence of the redistribution of the flow. These characteristics are explained by using a concept similar to that of Butterworth's spreading model. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(1): 47–60, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20095     

Critical heat flux for subcooled flow boiling in micro-channel heat sinks

Critical heat flux (CHF) was measured and examined with high-speed video for subcooled flow boiling in micro-channel heat sinks using HFE 7100 as working fluid. High subcooling was achieved by pre-cooling the working fluid using a secondary low-temperature refrigeration system. The high subcooling greatly reduced both bubble departure diameter and void fraction, and precluded flow pattern transitions beyond the bubbly regime. CHF was triggered by vapor blanket formation along the micro-channel walls despite the presence of abundant core liquid, which is consistent with the mechanism of Departure from Nucleate Boiling (DNB). CHF increased with increasing mass velocity and/or subcooling and decreasing hydraulic diameter for a given total mass flow rate. A pre-mature type of CHF was caused by vapor backflow into the heat sink’s inlet plenum at low mass velocities and small inlet subcoolings, and was associated with significant fluctuations in inlet and outlet pressure, as well as wall temperature. A systematic technique is developed to modify existing CHF correlations to more accurately account for features unique to micro-channel heat sinks, including rectangular cross-section, three-sided heating, and flow interaction between micro-channels. This technique is shown to be successful at correlating micro-channel heat sink data corresponding to different hydraulic diameters, mass velocities and inlet temperatures.     

Experimental Investigation on Critical Heat Flux for Water Flowing in a Vertical Uniformly Heated Rifled Tube under Near-critical Pressures

This study experimentally investigated the critical heat flux(CHF) of departure from nucleate boiling(DNB) of water flowing under near-critical pressures in a 2 m-long vertical upward rifled tube with the size of Φ35 × 5.67 mm. Operating conditions included pressures of 18–21 MPa, mass fluxes of 475–1000 kg·m~(-2)·s~(-1), inlet subcooling temperatures of 3–5°C, and wall heat fluxes of 40–960 kW·m~(-2). Tube wall temperature distribution and heat transfer performance in different test conditions were obtained. The effects of the operating parameters on CHF were analyzed. Four heat transfer coefficient correlations were evaluated against our experimental data for further investigation of the two-phase heat transfer characteristics. A heat transfer correlation based on Martinelli number utilized in two-phase region and two empirical correlations used to predict the CHF in rifled tube at near-critical pressures were proposed. Meanwhile, experimental CHF data in rifled tube were compared with six widely used correlations and a CHF look-up table. The CHF enhancement effect in rifled tube is obvious as compared with the CHF data in smooth tube. Results show that DNB occurs at low vapor quality and subcooled region in the rifled tube at near-critical pressures. The increase in pressure leads to the early occurrence of DNB and the decrease in CHF, whereas the increase in mass flux delays the occurrence of DNB and results in the increase in CHF. DNB presents a tendency to move toward the inlet of the rifled tube. At individual operating conditions, DNB and dryout coexist at different sections of the rifled tube.     

A new correlation for the limiting quality of critical heat flux at low mass fluxes and low pressures

The limiting quality (or limiting critical quality) was studied experimentally. We used a test section made of Inconel‐600 with 8 mm OD, 170 mm length and 1 mm wall thickness. The experiments were carried out with water at up flow for mass fluxes from 100 to 400 kg m^?2 s^?1, at low pressures (1.0–7.0 bar) and subcoolings (up to 70 K). The limiting quality is characterized by a sharp drop of the critical heat flux (CHF) in a diagram CHF versus critical quality with fixed values of pressure and mass flux and with change of the inlet subcooling. It was observed that the limiting quality appeared in our experimental results. There is a disagreement in the literature on how this phenomenon occurs. According to the present study the limiting quality phenomenon takes place due to a change of CHF mechanisms between DNB and dryout in agreement with Doroshchuk's findings. Existent correlations from literature have been compared with the experimental data. However, no correlations agree with our data because of their different validity ranges. Therefore a new correlation for the limiting quality phenomenon is developed for a large range of pressure. Furthermore the critical heat flux is calculated via a heat balance in which the value of the limiting quality is included. Copyright © 2004 John Wiley & Sons, Ltd.     

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.     

Dry-out CHF correlation for R134a flow boiling in a horizontal helically-coiled tube

An experimental study was carried out to investigate the R134a dry-out critical heat flux (CHF) characteristics in a horizontal helically-coiled tube. The test section was heated uniformly by DC high-power source, and its geometrical parameters are the outer diameter of 10 mm, inner diameter of 8.4 mm, coil diameter of 300 mm, helical pitch of 75 mm and valid heated length of 1.89 m. The experimental parameters are the outlet pressures of 0.30–0.95 MPa, mass fluxes of 60–500 kg m^?2 s^?1, inlet qualities of ?0.36–0.35 and heat fluxes of 7.0 × 10³–5.0 × 10⁴ W m^?2. A method based on Agilent BenchLink Data Logger Pro was developed to determine the occurrence of CHF with a total of 68 T-type thermocouples (0.2 mm) set along the tube for accurate temperature measurement. The characteristics of wall temperatures and the parametric effect on dry-out CHF showed that temperature would jump abruptly at the point of CHF, which usually started to form at the front and offside (270° and 90°) of the outlet cross-section. The CHF values decrease nearly linearly with increasing inlet qualities, while they decrease more acutely with increasing critical qualities, especially under larger mass flux conditions. The mass flux has a positive effect on CHF enhancement, but the pressure has negative one. A new dimensionless correlation was developed to estimate dry-out CHF of R134a flow boiling in horizontal helically-coiled tubes under current experimental conditions and compared to calculated results from Bowring and Shah correlations.     

Experimental study of CHF in a vertical spirally internally ribbed tube under the condition of high pressures

In the present paper, experiments of CHF were, respectively, conducted in a smooth tube and a four-head spirally internally ribbed tube. The smooth tube has an average inner diameter of 11.69 mm (∅18×3 mm). The four-head spirally internally ribbed tube has an average inner diameter of 11.26 mm (∅22×5.5 mm). The test section was vertically installed in the test loop and was uniformly heated by electricity. The working fluid was water, which entered into the test tube upward with an inlet subcooled temperature of 40–50 °C. The test pressure ranged from 10 to 21 MPa. The mass flux ranged from 400 to 1600 kg·m⁻²·s⁻¹. The effects of various parameters such as mass flux and pressure on CHF are presented. The experimental results in the smooth tube and the four-head spirally internally ribbed tube are compared with each other. It shows that CHF can be enhanced by the four-head spirally internally ribbed tube in the test range. The experimental data in the smooth tube are used to evaluate a precise CHF correlation available in the literature. Good agreements are obtained.     

Flow boiling CHF enhancement with surfactant solutions under atmospheric pressure

Surfactant effect on CHF (critical heat flux) was determined during water flow boiling at atmospheric pressure in closed loop filled with solution of tri-sodium phosphate (TSP, Na₃PO₄ · 12H₂O). TSP was added to the containment sump water to adjust pH level during accident in nuclear power plants. CHF was measured for four different water surfactant solutions in vertical tubes, at different mass fluxes (100–500 kg/m² s) and two inlet subcooling temperatures (50 °C and 75 °C). Surfactant solutions (0.05–0.2%) at low mass flux (～100 kg/m² s) showed the best CHF enhancement. CHF was decreased at high mass flux (500 kg/m² s) compared to the reference plain water data. Maximum increase in CHF was about 48% as compared to the reference data. Surfactant caused a decrease in contact angle associated with an increase of CHF from surfactant addition.     

An experimental study on CHF enhancement in flow boiling using Al2O3 nano-fluid

The critical heat flux (CHF) is one of the most important thermal hydraulic parameters in heat transfer system design and safety analyses. CHF enhancement allows higher limits of operation conditions such that heat transfer equipment can be operated safely with greater margins and better economy. The application of nano-fluids is thought to have strong potential for enhancing the CHF. In this study, zeta potentials of Al₂O₃ nano-fluids were measured and flow boiling CHF enhancement experiments using Al₂O₃ nano-fluids were conducted under atmospheric pressure. The CHFs of Al₂O₃ nano-fluids were enhanced up to ～70% in flow boiling for all experimental conditions. Maximum CHF enhancement (70.24%) was shown at 0.01 vol% concentration, 50 °C inlet subcooling, and a mass flux of 100 kg/m² s. Inner surfaces of the test section tube were observed by FE–SEM and the zeta potentials of Al₂O₃ nano-fluids were measured before and after the CHF experiments.     

Film boiling heat transfer around a vertical finite‐length cylinder with a convex hemispherical bottom

The film boiling heat transfer around a vertical silver cylinder with a convex hemispherical bottom was investigated experimentally in quiescent water at atmospheric pressure. The experiments have been carried out using a quenching method. The diameter and length of the test cylinder are 32 mm and 48 mm, respectively. The test cylinder was heated to about 600 °C in an electric furnace and then cooled in saturated or subcooled water with an immersion depth of about 100 mm. The degree of liquid subcooling was varied from 0 K to 30 K. The analytical solutions for saturated and subcooled boiling are obtained by applying a two‐phase boundary layer theory for vapor film with a smooth interface. The experimental data correlates within ±15% based on the proposed prediction method. Also, the lower limit of film boiling was examined in terms of wall heat flux and degree of superheating. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20289     

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.     

Subcooled flow boiling CHF enhancement with porous surface coatings

The effect of micro/nanoporous inside surface coated vertical tubes on CHF was determined during water flow boiling at atmospheric pressure. CHF was measured for smooth and three different coated tubes, at mass fluxes (100–300 kg/m² s) and two inlet subcooling temperatures (50 °C and 75 °C). Greater CHF enhancement was found with microporous coatings than with nanoporous coatings. Al₂O₃ microporous coatings with particle size <10 μm and coatings thickness of 50 μm showed the best CHF enhancement. Maximum increase in CHF was about 25% for microporous Al₂O₃. A wettability test was performed to study an increase of CHF with microporous coated surfaces.     

Pressure drop studies on two-phase flow in a uniformly heated vertical tube at pressures up to the critical point

Measurements of two-phase flow pressure drop have been made during a phase-change heat transfer process with refrigerant (R-134a) as a working fluid for a wide range of pressures right up to the critical pressure. The experiments were conducted in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length over a heat flux range of 35–80 kW/m², mass flux range of 1200–2000 kg/m² s, exit quality range of 0.19–0.81 and for reduced pressures ranging from 0.24 to 1 with a fixed inlet subcooling of 3 °C. The measurements were compared with the predictions from the homogeneous flow model, a separated flow model using correlations drawn from the literature for void fraction and frictional pressure drop, and finally, using a flow pattern-based predictive method accounting specifically for bubbly, slug and annular flow regimes. It was found that the best results were obtained with the flow pattern-based approach with a mean deviation of ±20% over the entire pressure range.     

Study of the critical heat flux condition with water and R-123 during flow boiling in microtubes. Part II – Comparison of data with correlations and establishment of a new subcooled CHF correlation

This study’s objective was to better understand the CHF condition in microchannels. The effect of different operating parameters – mass flux, inlet subcooling, exit quality, heated length and diameter – were assessed in detail in Part I of the study and compared to the behavior in conventional sized channels. Part II of the study compares the water and R-123 data with existing micro/macrochannel correlations. Existing correlations for predicting CHF in large-sized channels do not seem to be applicable to microchannels. This study has provided new subcooled CHF data for low mass fluxes and the earlier available subcooled boiling CHF correlation for microchannels (based on the data available for very high mass fluxes) is not suitable to predict such data. Based on the new subcooled CHF data, a correlation to predict CHF in low-flow subcooled boiling has been developed.