CHF in a circumferentially non‐uniformly heated tube under low‐pressure and low‐mass‐flux condition (inclined upward flow)

Most investigations on forced convective boiling have been conducted by using uniformly heated round tubes under a vertical upward flow condition, although the actual system has a non‐uniformly heated condition with several tube orientations. The non‐uniformity of the heat flux and tube inclination causes the liquid film distribution, which in turn affects the critical heat flux. In this investigation, the flow and heat‐transfer characteristics were experimentally investigated under non‐uniformly heated conditions along the circumferential direction with a 45° tube inclination. In the experiment, CHF was measured by using two different heated lengths, i.e., 900 and 1800 mm. The experimental results showed a unique tendency of CHF caused by the interrelationship of the non‐uniform heat flux distribution, the tube inclination, and liquid film redistribution. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20333     

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     

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 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     

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     

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

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

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.     

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.     

Critical heat flux enhancement in pool‐boiling heat transfer using oxidized multi‐wall carbon nanotubes

In this study, the critical heat flux (CHF) and heat‐transfer coefficient under the pool‐boiling state were tested using multi‐wall carbon nanotubes (MWCNT) CM‐95, CM‐100, and oxidized MWCNT CM‐100. The results showed that the highest CHF increase for both MWCNT CM‐95 and CM‐100 was at the volume fraction of 0.001%, and that the CHF increase ratio for MWCNT CM‐100 nanofluid with long particles was higher than that for MWCNT CM‐95 nanofluid with short particles. In addition, at the volume fraction of 0.001%, the oxidized MWCNT CM‐100 nanofluid indicated a 47.27% higher CHF‐increase ratio as well as an approximately 21.04% higher heat‐transfer coefficient increase ratio compared with the MWCNT CM‐100 nanofluid without oxidation treatment. Copyright © 2015 John Wiley & Sons, Ltd.     

Effect of aspect ratio on saturated boiling flow in microchannels with nonuniform heat flux

Microchannel two‐phase flow is an effective cooling method used in microelectronics, in which the heat flux density is unevenly distributed usually. The paper is focused on numerical study the effect of aspect ratio on the flow boiling of microchannels with nonuniform heat flux. The heat source is a three‐dimensional (3D) integrated circuit. 3D microchannel model and volume of fluid method are coupled in numerical simulation. The results show that the aspect ratio has no relationship with the two‐phase pressure drop of the microchannel. It has a certain influence on the distribution of bubble shape. In terms of the heat transfer coefficient, the aspect ratio has a certain influence on a section of the inlet. Due to the nucleate boiling, the convective heat transfer in the remaining areas is the dominant factor and the average heat transfer coefficient is mainly determined by the heat flux at the bottom of the channel.     

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     

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     

High heat flux removal using water subcooled flow boiling in a single-side heated circular channel

High heat flux removal from plasma-facing components and electronic heat sinks involves conjugate heat transfer analysis of the applicable substrate and flowing fluid. For the present case of subcooled flow boiling inside a single-side heated circular channel, the dimensional results show the significant radial, circumferential and axial variations in all thermal quantities for the present radial aspect ratio (R_o=outside radius to inside radius) of 3.0. A unified, dimensionless representation of the two-dimensional inside wall heat flux, and the dimensional inside wall heat flux (q_i(φ,z)) and temperature (T_i(φ,z)) data was found and used to collapse the data for all circumferential locations. Finally, 2-D boiling curves are presented and are among the first full set of 2-D boiling data presented for a single-side heated circular configuration.     

A theoretical study of evaporative heat transfer in high‐velocity two‐phase flow of air–water in a small vertical tube

A theoretical study was performed to investigate the evaporative heat transfer of high‐velocity two‐phase flow of air–water in a small vertical tube under both heating conditions of constant wall temperature and constant heat flux. A simplified two‐phase flow boundary layer model was used to evaluate the evaporative heat transfer characteristics of the annular two‐phase flow. The analytical results show that the gravitational force, the gas–liquid surface tension force, and the inertial force are much smaller than the frictional force and hence can be neglected for a small tube. The evaporative heat transfer characteristics of the small tube with constant wall temperature are quite close to those of the small tube with constant heat flux. The mechanism of the heat transfer enhancement is the forced convective evaporation on the surface of the thin liquid film. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 430–444, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10110     

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     

环形加热炉内管坯辐射热流均匀性的分析

管坯在环形加热炉内加热时,其表面辐射热流的分布是很不均匀的,靠近炉底的管坯表面的辐射热流比上部的小得多,因而造成管坯表面自下向上温度分布不均匀。适当增加管坯间距与其半径比(一般在3～4较为适宜),以及提高炉底反射率,可提高管坯表面热流分布的均匀性,改善加热质量。     

Numerical simulation of steam–water two‐phase flow under dynamic load

A separated‐phase physical model for steam–water two‐phase flow on a rotating platform was developed. The mesh generation for a horizontal pipe was conducted, and the finite volume method was used to discretize the equations. Equations were solved with the SIMPLE algorithm after setting the initial and boundary conditions. Predicted results were compared with experimental data, and they agreed well with each other. The results showed that the fluid outlet pressure and pressure drop in the test section increased with increasing dynamic load. However, the effective heat transferred to the fluid decreased with the increase of dynamic load. The developed model can be used to simulate the gas–liquid two‐phase flow under different gravity or rotary conditions.     

Research on a new heat transfer model for liquid film convective evaporation in annular flow

Based on the phenomenon of turbulence restraint in liquid‐vapor interface, an analytical model is proposed for annular flow with a velocity distribution. The liquid‐vapor interface affecting district mixing length model was amended, and a new liquid film convective evaporation heat transfer model at the annular flow was developed. Compared with the experimental data, the results show that the new model is better than the model based on full tube flow velocity distribution. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 524–530, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10051     

Analysis of characteristics of water flash evaporation under low‐pressure conditions

The relation between the maximum heat transfer coefficient and the saturation temperature of vapor for water flash evaporation under low‐pressure conditions is theoretically analyzed. The optimum evaporation time is introduced by analytical consideration to reflect the experimental result of the linearity of the super heat flux and the unknown constant included in the optimum evaporation time is determined by the experiment. The analytical result is compared with experimental data and shows a good agreement. The analysis also gives information regarding the optimum spray condition among the spray flow rate, particle diameter, and the intermittent spray time to achieve the maximum heat transfer coefficient under low‐pressure conditions. © 1999 Scripta Technica, Heat Trans Asian Res, 29(1): 22–33, 2000     

Estimation of non‐equilibrium parameters of post‐dryout boiling processes of water

A thermodynamic study of non‐equilibrium, two‐phase flow processes has been presented, introducing the ‘non‐equilibrium parameters’ related with the causes for deviations from equilibrium and the advance of relaxation phenomena towards equilibrium. Taking these non‐equilibrium parameters as input, a comparative parametric study of several processes may be realized and indices relative to the quality of these processes, like entropy production, temperature deviation, etc., may be obtained as output. In the present work an estimation of non‐equilibrium parameters is undertaken for the specific pattern of water boiling in the post dry‐out region, using available data for the vapour superheating. Copyright © 1999 John Wiley & Sons, Ltd.