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.     

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…     

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.     

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 R22 and R142b mixtures flowing in a vertical tube

An experimental study on the critical heat flux (CHF) in forced convective boiling of R142b/R22 mixtures was conducted with an electrically heated vertical test tube with 17 mm inner diameter, in which the inner surface temperature was controlled and the heated length varied from 0.058 to 0.661 m. Experiments were carried out under the following conditions: constant pressures of 1.3 and 2.0 MPa, constant mass fluxes of 1000 and 2000 kg/(m²s), and constant outlet qualities of −0.2, 0.0, and 0.1. Whole boiling curves were obtained and the effects of heated length, local quality, and mole fraction on the CHF were investigated. The CHF of mixtures was higher than that of pure fluids and showed a maximum in a CHF versus mole fraction plot at a certain mole fraction. However, the maximum was less significant for the longer test section and the higher outlet quality condition. © 1998 Scripta Technica. Heat Trans Jpn. Res., 26(5): 292–305, 1997     

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     

Prediction of liquid hydrogen flow boiling critical heat flux condition under microgravity based on the wall heat flux partition model

Critical heat flux (CHF) of liquid hydrogen (LH2) flow boiling under microgravity is vital for designing space cryogenic propellant conveying pipe since the excursion of wall temperature may cause system failure. In this study, a two-dimensional axisymmetric model based on the wall heat flux partition (WHFP) model was proposed to predict the CHF condition under microgravity including the wall temperature and the CHF location. The proposed numerical model was validated to demonstrate a good agreement between the simulated and experimentally reported results. Then, the wall temperature distribution and the CHF location under different gravity conditions were compared. In addition, the WHFP and vapor-liquid distribution along the wall under microgravity were predicted and its difference with terrestrial gravity condition was also analysed and reported. Finally, the effects of flow velocity and inlet sub-cooling on the wall temperature distributions were analysed under microgravity and terrestrial gravity conditions, respectively. The results indicate that the CHF location moves upstream about 5.25 m from 1g to 10⁻⁴g since the void fraction near the wall reaches the breakpoint of CHF condition much earlier under the microgravity condition. Furthermore, the increase of the velocity and decrease of the sub-cooling have smaller effects on the CHF location during LH2 flow boiling under microgravity.     

Experimental study on convective boiling heat transfer in vertical narrow-gap annular tube

IntroductionConvechve boiling or highly subcooled single-Phaseforced convention in micro-channels is an effeCtivecooling meChedsm with a wide ~ge of aPPlications.Among these are the COOling of such diverse system as. accelerator abets, high power resistive magnets,compact fission ~ cores, fusion ~ blankets,advanced space thermal management systems,manufachang and materials Processing OPerations, andhigh-density multi~chip modules in supe~ andOther modular eleCtronics. These devices involv…     

Effect of tube bundles on nucleate boiling and critical heat flux

In order to elucidate boiling heat transfer characteristics for each tube and the critical heat flux (CHF) for tube bundles, an experimental investigation of pool and flow boiling of Freon-113 at 0.1 MPa was performed using two typical tube arrangements. A total of fifty heating tubes of 14 mm diameter, equipped with thermocouples and cartridge heaters, were arrayed at pitches of 18.2 and 21.0 mm to simulate both square in-line and equilateral staggered bundles. For the flow boiling tests the same bundles as were used in pool boiling were installed in a vertical rectangular channel, to which the fluid was supplied with an approach velocity varying from 0.022 to 0.22 m/s. It was found in this study that the boiling heat transfer coefficient of each tube in a bundle was higher than that for an isolated single tube in pool boiling. This enhancement increases for tubes at higher locations, but decreases as heat flux is increased. At heat fluxes exceeding certain values, the heat transfer coefficient becomes the same as that for an isolated tube. As the heat flux approaches the CHF, flow pulsations occurred in the pool boiling experiments although the heat transfer coefficient was invariant even under this situation. The approach velocity has an appreciable effect on heat transfer up to a certain level of heat flux. In this range of heat flux, the heat transfer coefficient exceeds the values observed for pool boiling. An additive method with two contributions, i.e., single phase convection and boiling, was used to predict the heat transfer coefficient for bundles. The predicted results showed reasonable agreement with the measured results. The critical heat flux in tube bundles tended to increase as more bubbles were rising through the tube clearance. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 312–325, 1998     

Transient natural convection heat transfer of liquid D-mannitol on a horizontal cylinder

Transient and steady state natural convection heat transfer for D-mannitol on a horizontal cylinder was investigated experimentally at various liquid temperatures and heat input conditions. To clarify the natural convection phenomena of D-mannitol, transient and steady heat transfer coefficients were measured under various liquid temperatures of D-mannitol and periods of heat generation rates from a horizontal platinum cylinder. The platinum cylinder with a diameter of 1 mm and a length of 43.5 mm was used as the test heater in this experiment. Experimental results indicated that the steady heat transfer coefficient of D-mannitol was affected by the liquid temperature. As the liquid temperature increased, it was understood that the effect of liquid temperature weakened. When the period of the heat generation rate was changed, the heat transfer process was divided into natural convection heat transfer and conductive heat transfer. It was considered that the conductive heat transfer was more dominant as the period of the heat generation rate decreased. The empirical correlations of steady and transient heat transfer coefficients for D-mannitol were obtained.     

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     

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.     

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.     

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.     

Comparison of CHF measurements in horizontal and vertical tubes cooled with R-134a

An experimental study of the critical heat flux (CHF) in horizontal and vertical tubes cooled with R-134a has been completed. The investigated ranges of flow parameters in R-134a were outlet pressures of 1.31, 1.67 and 2.03 MPa (8, 10 and 12 MPa in water-equivalent values), mass flux from 500 to 3000 kg m⁻² s⁻¹ (700-4300 kg m⁻² s⁻¹ in water-equivalent values), and critical quality from −0.1 to +0.9. The wide range of qualities was achieved using tubes of different heated lengths and two-phase flow at the test-section inlet.The R-134a CHF data obtained in the vertical orientation agreed with the R-134a-equivalent CHF values from the water-based CHF look-up table. The effect of orientation on CHF was found to depend on mass flux, quality and pressure, as well as the limiting critical quality. This effect is strong at low mass fluxes, but disappears at high mass fluxes. At qualities higher than the limiting critical qualities, the CHF in horizontal flow can be greater than the corresponding value in vertical flow at the same critical quality conditions. A maximum reduction in CHF due to flow stratification was observed at qualities between the limiting critical qualities for horizontal and vertical flows.The orientation effect on CHF appears to be much stronger for R-134a than for water flow at the same critical quality, equivalent mass flux (based on vertical flow fluid-to-fluid modeling relationships) and density ratio. This behavior is primarily due to the larger density difference between liquid and vapor and the lower vapor velocity in R-134a.     

Boiling heat transfer characteristics of R-113 in a vertical small diameter tube under natural circulation condition

Boiling heat transfer characteristics of freon R-113 are experimentally investigated in a vertical small diameter tube, D=1.45 mm and L=100 mm at a wide pressure range of 19-269 kPa under natural circulation condition. Except the entrance region of the test section, the flow regime is annular in view of the measured vapor flux. The pool boiling correlations of Stephan and Abdelsalam and McNelly equally well predict the experimental data within an error of ±20%. No enhancement of heat transfer coefficient is obtained although D/B is less than 1.5, which differs from the finding of Klimenko.     

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     

Natural convection boundary layer on a horizontal elliptical cylinder with constant heat flux and internal heat generation

In this paper the natural convection boundary layer on a horizontal elliptical cylinder with constant heat flux and temperature dependent internal heat generation is investigated. The mathematical problem is reduced to a pair of coupled partial differential equations for the temperature and the stream function, and the resulting nonlinear equations are solved numerically by cubic spline collocation method. Results for the local Nusselt number and the local skin-friction coefficient are presented as functions of eccentric angle for various values of heat generation parameters, Prandtl numbers and aspect ratios. An increase in the aspect ratio of the elliptical cylinder decreases the average surface temperature of the elliptical cylinder with blunt orientation, while it increases the average surface temperature of the elliptical cylinder with slender orientation. Moreover, an increase in the heat generation parameter for natural convection flow over a horizontal elliptic cylinder with constant heat flux leads to an increase in the average surface temperature of the elliptical cylinder.     

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