Critical heat flux in subcooled flow boiling with water in a tube with axially nonuniform heating

Critical heat flux (CHF) in subcooled flow boiling under axially nonuniform heating conditions was experimentally investigated using a tube heated with a dc power source. The thickness of the tube wall in the axial direction was varied to attain axially nonuniform heating. The different thicknesses, therefore, separated the tube into regions of high heat flux and regions of low heat flux. The lengths of these regions of the tube were also varied to study the effect on the CHF. The objective of this system is to initiate boiling in the high-heat-flux region, thus increasing heat transfer, and to interrupt the bubble boundary layer in the low-heat-flux region. Because it is the initiation of boiling that increases heat transfer, the performance of such a system is linked to its effectiveness in repeatedly interrupting and re-establishing the bubble boundary layer. Our experiments, involving tubes that had sections of different thicknesses and different lengths, showed that when the heat flux in the low-heat-flux region was below the net vapor generation (NVG) heat flux, this system enhanced the CHF, but not when it was above the NVG. Also, for relatively short low-heat-flux regions, the CHF was not enhanced, presumably because there was insufficient time to interrupt the bubble boundary layer. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(2): 169–178, 1998     

Photographic study of high-flux subcooled flow boiling and critical heat flux

This study examines both high-flux flow boiling and critical heat flux (CHF) under highly subcooled conditions using FC-72 as working fluid. Experiments were performed in a horizontal flow channel that was heated along its bottom wall. High-speed video imaging and photomicrographic techniques were used to capture interfacial features and reveal the sequence of events leading to CHF. At about 80% of CHF, bubbles coalesced into oblong vapor patches while sliding along the heated wall. These patches grew in size with increasing heat flux, eventually evolving into a fairly continuous vapor layer that permitted liquid contact with the wall only in the wave troughs between vapor patches. CHF was triggered when this liquid contact was finally halted. These findings prove that the CHF mechanism for subcooled flow boiling is consistent with the interfacial lift-off mechanism proposed previously for saturated flow boiling.     

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     

High Heat Flux Burnout in Subcooled Flow Boiling

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Effect of heated length on the critical heat flux of subcooled flow boiling. Part 1: Observation of bubbles and slug length at atmospheric pressure

The effect of the heated length on the Critical Heat Flux (CHF) of subcooled flow boiling with water was experimentally investigated by means of a direct current heated plate made of stainless steel with various lengths. The motion of bubbles at near burnout was observed by a high‐speed video camera under atmospheric pressure conditions. Consideration was made of the critical wavelength of the Helmholtz instability in several CHF mechanistic models. The slug length at CHF almost coincided with the critical wavelength. It is further confirmed that when the heated length is shorter than the critical wavelength, a large enhancement of the CHF is seen. © 2000 Scripta Technica, Heat Trans Asian Res, 29(2): 132–143, 2000     

Critical heat flux enhancement in pool boiling using alumina nanofluids

The pool boiling characteristics of dilute dispersions of alumina nanoparticles in water were studied. Consistent with other nanofluid studies, it was found that a significant enhancement in critical heat flux (CHF) can be achieved at modest nanoparticle concentrations (<0.1% by volume). During experimentation and subsequent inspection, formation of a porous layer of nanoparticles on the heater surface occurred during nucleate boiling. This layer significantly changes surface texture of the heater wire surface which could be the reason for improvement in the CHF value. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20301     

Prediction of critical heat flux on natural convective boiling in vertical short‐thick tube submerged in saturated liquid

An experimental and semitheoretical study was carried out for the critical heat flux (CHF) on natural convective boiling in uniformly heated vertical short‐thick tubes and vertical short‐thick annular tubes submerged in saturated liquids. By adapting a mathematical dealing method based on the theoretical formulas of CHF of both the natural convective boiling in vertical narrow‐long tubes and the pool boiling, a simple semitheoretical formula was derived. The new formula expands the prediction range of CHF from pool boiling of vertical plates to very long vertical tubes and agrees well with the data of the tubes, annular tubes submerged in water or other liquids under various pressure conditions. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 402–410, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10103     

Critical Heat Flux of Boiling Heat Transfer in a Moderate Narrow Space

INTanDUCTI0NBoilingheattransferandcriticalheatflux(CHF)inaconfinednarrowspacehavebeenstudiedexperi-melltallybyanumberofinvestigatorsinthepastfewdecades.However,thereisnoanypopularlyacceptedmodelintheheattransferinnarrowspaceboiling,althoughsomepopularknowledgeabouttheboilingheattransferinthenarrowspacehavebeenacceptedbymanyresearchers.Theknowledgecanbecon-cludedasthatthenucleateboilingheattransferisenhancedatlowheatfluxregionanddeterioratedathighheatfiuxregi0nespeciallyatCHF.Theenhanceme…     

Temperature measurements near a heating surface at high heat fluxes in subcooled pool boiling

In previous papers (Int J Heat Mass Transfer, 2008;50:3481–3489, 2009;52: 814–821), the authors conducted measurements of liquid–vapor structures in the vicinity of a heating surface for subcooled pool boiling on an upward‐facing copper surface by using a conducting probe method. We reported that the macrolayer dryout model is the most appropriate model of the CHF and that the reason why the CHF increases with increasing subcooling is most likely that a thick macrolayer is able to form beneath large vapor masses and the lowest heat flux of the vapor mass region shifts towards the higher heat flux. To develop a mechanistic model of the CHF for subcooled boiling, therefore, it is necessary to elucidate the effects of local subcooling on boiling behaviors in the vicinity of a heating surface. This paper measured local temperatures close to a heating surface using a micro‐thermocouple at high heat fluxes for water boiling on an upward‐facing surface in the 0 to 40 K range of subcooling. A value for the effective subcooling, defined as the local subcooling during the period while vapor masses are being formed was estimated from the detected bottom peaks of the temperature fluctuations. It was established that the effective subcooling adjacent to the surface remains at considerably lower values than the bulk liquid subcooling. This suggests that, from nucleation to coalescence, the subcooling of a bulk liquid has a smaller effect on the behavior of primary bubbles than the extent of the subcooling would appear to suggest. An empirical correlation of the effective subcooling is proposed to provide a step towards quantitative modeling of the CHF for subcooled boiling. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20277     

Critical heat flux of countercurrent boiling in an inclined small tube with closed bottom

The study was focused on the effect of the inclination angle on the critical heat flux of countercurrent boiling in an inclined uniformly heated tube with open top and closed bottom ends at zero inlet flow. The experimental results show that the CHF data of the small vertical tubes agree reasonably well with the predicting correlation proposed by Tien. The CHF data of the small inclined tubes decrease with reducing the inclination angle. The experimental data of the inclined tubes agrees reasonably well with the modified correlation, which is resulted from the conventional correlation for vertical tubes.     

Visualization of a principle mechanism of critical heat flux in pool boiling

A new experimental work was made to discover a principle mechanism of the burnout in pool boiling. Here, we directly observed a liquid layer structure under a massive vapor clot and the liquid layer-related burnout phenomenon. Based on the present observations, we have made a visual model for the formation and dryout of a liquid film under its vapor environment. At the formation process, liquid is trapped in interleaved space between growing bubbles and surface and the liquid trapping continues between coalesced bubbles and surface. In the dryout process, we especially observed vapor “holes” made by spontaneous breakup of discrete nucleating bubbles inside a vapor clot. The burnout can be triggered by the evaporation of the liquid film region expanded from rims of the holes.     

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     

Compound heat transfer enhancement of a dimpled tube with a twisted tape swirl generator

Friction and compound heat transfer behaviors in a dimpled tube fitted with a twisted tape swirl generator are investigated experimentally using air as working fluid. The effects of the pitch and twist ratio on the average heat transfer coefficient and the pressure loss are determined in a circular tube with the fully developed flow for the Reynolds number in the range of 12,000 to 44,000. The experiments are performed using two dimpled tubes with different pitch ratios of dimpled surfaces (PR = 0.7 and 1.0) and three twisted tapes with three different twist ratios (y/w = 3, 5, and 7). Experiments using plain tube and dimpled tube acting alone are also carried out for comparison. The experimental results reveal that both heat transfer coefficient and friction factor in the dimpled tube fitted with the twisted tape, are higher than those in the dimple tube acting alone and plain tube. It is also found that the heat transfer coefficient and friction factor in the combined devices increase as the pitch ratio (PR) and twist ratio (y/w) decrease. In addition, an empirical correlation based on the experimental results of the present study is sufficiently accurate for prediction the heat transfer (Nu) and friction factor (f) behaviors.     

The effect of heating direction on flow boiling heat transfer of R134a in micro-channels

This paper presents effects of heating directions on heat transfer performance of R134a flow boiling in micro-channel heat sink.The heat sink has 30 parallel rectangular channels with cross-sectional dimensions of 500μm width 500μm depth and 30mm length.The experimental operation condition ranges of the heat flux and the mass flux were 13.48 to 82.25 W/cm2 and 373.3 to 1244.4 kg/m2s respectively.The vapor quality ranged from 0.07 to 0.93.The heat transfer coefficients of top heating and bottom heating both were up to 25 kW/m2 K.Two dominate transfer mechanisms of nucleate boiling and convection boiling were observed according to boiling curves.The experimental results indicated that the heat transfer coefficient of bottom heating was 13.9％ higher than top heating in low heat flux,while in high heat flux,the heat transfer coefficient of bottom heating was 9.9％.higher than the top heating,because bubbles were harder to divorce the heating wall.And a modified correlation was provided to predict heat transfer of top heating.     

Critical heat flux under conditions of high subcooling and high velocity at atmospheric pressure

A quantitative analysis of critical heat flux (CHF) under high mass flux with high subcooling at atmospheric pressure was successfully carried out by applying a new transition region model for a macro-water sublayer on heated walls to the existing model of a vapor blanket over the macro-water sublayer. The CHF correlation proposed in this study could predict well the experimental data obtained for water mass flux of 940 to 20,300 kg/m²s using circulate tubes 2 to 4 mm in diameter and 30 to 100 mm in length with inlet subcooling of 30 to 90 °C and rectangular channels heated from one side with gaps of 3 to 20 mm, length of 50 to 305 mm, and inlet subcooling of 30 to 77 °C and revealed a unique feature of CHF, namely, that the effects of wall friction of subcooled boiling flow and the velocity of the steam blanket above the macro-water sublayer at atmospheric pressure become the dominant factors while they were not dominant at higher pressures. © 1997 Scripta Technica, Inc Heat Trans Jpn Res, 26 (1): 16–29, 1997     

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     

Flow boiling of liquid nitrogen in micro-tubes: Part II – Heat transfer characteristics and critical heat flux

This paper is the second portion of a two-part study concerning the flow boiling of liquid nitrogen in the micro-tubes with the diameters of 0.531, 0.834, 1.042 and 1.931 mm. The contents include the heat transfer characteristics and critical heat flux (CHF). The local wall temperatures are measured, from which the local heat transfer coefficients are determined. The influences of heat flux, mass flux, pressure and tube diameter on the flow boiling heat transfer coefficients are investigated systematically. Two regions with different heat transfer mechanism can be classified: the nucleate boiling dominated region for low mass quality and the convection evaporation dominated region for high mass quality. For none of the existed correlations can predict the experimental data, a new correlation expressed by Co, Bo, We, K_p and X is proposed. The new correlation yields good fitting for 455 experimental data of 0.531, 0.834 and 1.042 mm micro-tubes with a mean absolute error (MAE) of 13.7%. For 1.931 mm tube, the flow boiling heat transfer characteristics are similar to those of macro-channels, and the heat transfer coefficient can be estimated by Chen correlation. Critical heat flux (CHF) is also measured for the four tubes. Both the CHF and the critical mass quality (CMQ) are higher than those for conventional channels. According to the relationship that CMQ decreases with the mass flux, the mechanism of CHF in micro-tubes is postulated to be the dryout or tear of the thin liquid film near the inner wall. It is found that CHF increases gradually with the decrease of tube diameter.     

Critical heat flux performance for flow boiling of R-134a in vertical uniformly heated smooth tube and rifled tubes

In the present paper, critical heat flux (CHF) experiments for flow boiling of R-134a were performed to investigate the CHF characteristics of four-head and six-head rifled tubes in comparison with a smooth tube. Both of rifled tubes having different head geometry have the maximum inner diameter of 17.04 mm while the smooth tube has the average inner diameter of 17.04 mm. The experiments were conducted for the vertical orientation under outlet pressures of 13, 16.5, and 23.9 bar, mass fluxes of 285-1300 kg/m²s and inlet subcooling temperatures of 5-40 °C in the R-134a CHF test loop. The parametric trends of CHF for the tubes show a good agreement with previous understanding. In particular, CHF data of the smooth tube for R-134a were compared with well-known CHF correlations such as Bowring and Katto correlations. The CHF in the rifled tube was enhanced to 40-60% for the CHF in the smooth tube with depending on the rifled geometry and flow parameters such as pressure and mass flux. In relation to the enhancement mechanism, the relative vapor velocity is used to explain the characteristics of the CHF performance in the rifled tube.