Critical Heat Flux Measurements Research Articles

비정상 열전도 역산법에 의한 분무냉각 임계열유속(CHF)의 측정에 관한 연구

비정상 열전도 역산법에 의한 분무냉각 임계열유속(CHF)의 측정에 관한 연구

Guest Editorial: Special Issue on Polyurethane Foam Combustion

Flexible and rigid polyurethane foams can be formulated with a wide variety of mechanical, thermal, chemical and physical properties. Since they are lightweight and can be engineered for specific purposes, they are finding increasing application in the world around us—the construction, insulation, furniture and transportation industries. Yet, hydrocarbon based foams are very flammable, so fire safety and fire performance are key factors for many applications, as well as for their long term use in most sectors. As such, multi-faceted research is underway in many countries with focus on foam fire behaviour, application and scaling of fire test results and product safety of polyurethane foams and related products. The purpose of this special section of Fire Technology is to communicate to the broad fire protection community a selection of interesting research findings and possible future advances related to the fire science and engineering of polyurethane foams. Three papers, describing both experimental and numerical investigations, are included in the section. As well as presenting new research findings, the papers address the implications of the results to product design and regulation, as well as fire test standard development. Authors representing research organizations and universities in Canada, the United Kingdom and the United States have contributed to this special section. The three papers compiled here deal with flexible polyurethane foam, which is used extensively in upholstered furniture, mattresses and other consumer products. Pitts [1] describes the detailed burning behavior of polyurethane foam using data from cone calorimeter tests, and discusses key issues associated with using the cone calorimeter to measure heat release rates of the foam. Hadden et al. [2] then examine the effects of the size of cone calorimeter specimens on measurements of critical heat flux for smoldering and flaming ignition, as well as temperatures and flame spread rates during smoldering of the foam. Finally, Ezinwa et al. [3] discuss the ability of two fire safety engineering models to predict full-scale fire behavior of foam slabs using information from cone calorimeter tests of the foam specimens. While these three papers cover only a small cross-section of issues related to fire safety testing of polyurethane foams, it is hoped that they generate discussion and interest leading to continued development of fire safe products and the associated performance test standards.

Experimental Measurements of Critical Heat Flux in Expanding Microchannel Arrays

The effect of an expanding microchannel cross-section on flow boiling critical heat flux (CHF) is experimentally investigated across four rates of expansion. A pumped-loop apparatus is developed to boil R-134a in an array of microchannels cut into copper; a test section is designed to facilitate interchange of the microchannel specimens, allowing consistency across experiments. An optimum expansion angle allowing maximum heat flux is observed, the location of which increases with the mass flow rate. The boiling number does not indicate any optimum in the range observed, showing a nearly monotonic increase with expansion angle. The familiar increase in critical heat flux with mass flux is observed, though expansion shifts the CHF-mass flux curves in a favorable direction. The existence of an optimum expansion angle confirms an earlier qualitative hypothesis by the authors and suggests that microchannel heat sinks offer opportunities for methodical improvement of flow boiling stability and performance.

Investigations on the pool boiling heat transfer and critical heat flux of ZnO-ethylene glycol nanofluids

Abstract Surfactant free and fairly stable ZnO-ethylene glycol (EG) nanofluids are prepared using prolonged sonication (>60 h). Extended period of sonication results in superior fragmentation and dispersion of ZnO nanoparticles, as is evident from the DLS data. Thermal conductivity of the prepared nanofluid displays a maximum enhancement of 40% for 3.75% volume fractions of ZnO loading in EG at 30 °C. The nucleate pool boiling heat transfer characteristics of ZnO-EG nanofluids with various loading of ZnO nanoparticles are measured at atmospheric pressure employing a cylindrical polished copper heater surface. The boiling heat transfer coefficient enhances with ZnO concentration and attains a maximum of 22% compared to that of base fluid for ZnO volume fraction of 1.6%. However, further loading of ZnO nanoparticles in EG decreases the heat transfer coefficient. Critical heat flux measurements are performed with a thin Constantan wire. CHF value appreciably increases with increasing ZnO loading and displays a maximum enhancement of ∼117% for nanofluid containing 2.6% volume fractions of ZnO.

Measurements of critical heat flux and liquid–vapor structure near the heating surface in pool boiling of 2-propanol/water mixtures

Saturated pool boiling of 2-propanol/water mixtures on a 12 mm diameter horizontal disk under atmospheric pressure was investigated. The CHF of the mixtures increased up to 1.7 times the CHF of water at 3.0–4.7 mol% concentrations of 2-propanol. To examine the mechanism of the CHF enhancement in the mixtures, liquid–vapor structures close to the heating surface were measured using a conductance probe. It was found that in the boiling of the mixtures, liquid–vapor structures show strong non-uniformity in the radial direction of the heating surface. The void fractions at 0.1–1 mm above the heating surface are small at the central region and large near the periphery of the heating surface. The liquid layer between the vapor mass and the heating surface is considerably thicker than that of water at the central region and becomes thinner near the periphery of the heating surface. This thicker liquid layer is likely to be the cause of the CHF enhancement in the 2-propanol/water mixtures.

Measurements of critical heat flux in CANDU 37-element bundle with a steep variation in radial power profile

An experiment has been performed to obtain dryout power measurements with a 37-element bundle string simulating radial power profiles of high-enriched fuel (which contains slightly higher enrichment than the CANDU fuel and is not the same as the traditional highly enriched uranium) and natural-uranium fuel. The electrically heated bundle string was cooled with Refrigerant-134a and installed vertically inside the test station. Occurrences of CHF were detected using sliding thermocouples installed inside each element. Measurements showed that the dryout power for the high-enriched fuel bundle is lower than that for the natural-uranium fuel bundle by 26%, on average. Similarly, the corresponding critical heat flux (CHF) values for the high-enriched fuel power profile are lower than those for the natural-uranium fuel power profile by about 48%. A correlation previously proposed for the effect of radial power profile on CHF underpredicts considerably the CHF for the high-enriched fuel power profile. The correlation has been revised to improve the prediction accuracy. The revised correlation represents closely the available database. It exhibits a correct asymptotic trend and hence can be extended to bundles with more severe variation in radial power profile than those covered in the current experiment.

Orientation and Related Buoyancy Effects in Low‐velocity Flow Boiling

This work is an extension of experimental results reported previously, which might provide design guidance for approximating certain aspects of the flow boiling process in microgravity but taking place in Earth gravity. In that research the buoyancy effects on the bubble dynamics were minimized by the imposition of a liquid velocity parallel to a flat heater surface in the inverted horizontal position, or nearly horizontal (within +/-5 degrees ), thus holding the heated liquid and vapor formed close to the heater surface. For the fluid used, liquid velocities in the range U= 5-10 cm/s were judged to be critical for changes in the behavior of the flow boiling process. Using the hydraulic diameter of the rectangular duct used, with the heater surface embedded in one side, this velocity range gives rise to flow Reynolds numbers on the order of 4400-8800. It was subsequently judged to be of interest to extend the range of orientation of the flat heater surface relative to gravity to the full circular range of 0-360 degrees, in increments of 45 degrees, and the results of this work are presented here. A solid massive copper heater with a gold-plated boiling heat transfer surface 19 x 38 mm in size, previously used for critical heat flux measurements with boiling, provided a near-uniform surface temperature. Only steady measurements of heat flux and surface temperature were possible with the copper heater. R-113 was the fluid used; the velocity was varied over the interval of 4-28 cm/s; bulk liquid subcooling was varied over 5-11 degrees C; and heat flux varied over 0-10 w/cm(2).

Boiling behaviors and critical heat flux on a horizontal plate in saturated pool boiling of water at high pressures

Observations of boiling behaviors and measurements of critical heat flux (CHF) were carried out for saturated water boiling on a horizontal, upward-facing plate at pressures from atmospheric to 7 MPa. The primary bubbles diminish in size almost in inverse proportion to pressure and commence to coalesce in the very low heat flux region. The diameter of detached coalesced bubbles increases with increases in the heat flux and reaches about 10 mm even at a pressure of 5 MPa. Detachment frequency of the coalesced bubbles was unaffected by the heat flux and pressure. The CHF predicted based on the macrolayer dryout model agrees well with the measured data.

Critical Heat Flux on a Horizontal Plate in Saturated Pool Boiling at High Pressure

Observation of boiling behavior and measurement of critical heat flux (CHF) were carried out for saturated water boiling on a horizontal, upward-facing plate at pressures from atmospheric to 7 MPa. The heating surface was made of nichrom with 20μm in thickness, 4 mm in width, and 27.5 mm in length. The primary bubbles diminish in size approximately with the inverse of the pressure and commence to coalesce in the very low heat flux region to form coalesced bubbles. The detached coalesced bubbles continuously increase in size with increases in the heat flux and grow to about 10 mm in equivalent diameter even at high pressure of 5 MPa. Detachment frequency of the coalesced bubbles was insensitive to the heat flux and pressure. The CHF was predicted based on the macrolayer dryout model where an empirical correlation of detachment frequency derived in the present paper and a correlation of macrolayer thickness previously proposed by the author and a coworker were used. The predicted CHF agrees fairly well with the measured values.

Measurement of critical heat flux for narrow annuli submerged in saturated water

An experimental study has been performed to improve understanding of boiling phenomena and critical heat flux (CHF) in narrow gaps. The experiment has been performed with annular test sections submerged in a pool of saturated water under atmospheric pressure with the following conditions: outer diameter of inner tube of 19 mm; heated length of 200 mm; annular gap sizes of 0.5–3.5 mm, and inclination angles from horizontal to vertical positions. The experimental results reveal that the CHF increases with the gap size and the inclination angle. A one-dimensional two-phase model with a modified friction factor correlation is suggested to predict the CHF condition. The developed model is also compared with other CHF data.

Saturated chf in horizontal eccentric annuli for low mass fluxes: measurement and modelling

Measurements of critical heat flux (CHF) were performed for a horizontal annular channel (diameters 11.2 and 54 mm, approx. 1 m heated length) in which the eccentricity of the inner heater was varied. Measurements for low mass fluxes (up to 1600 kg/m 2/s) using freon-12 at a pressure of approx. 10.3 bar and saturation inlet conditions show a strong influence of eccentricity on CHF. The influence of the mass flux on CHF is only moderate. The data trend is discussed on a basis of the flow pattern under which the phenomenon occurs. A model based on the analysis of an intermittent flow pattern was developed. The model explains satisfactorily the trend of the experimental data.

Critical heat flux measurements in a round tube for CFCs and CFC alternatives

Critical heat flux (CHF) measurements have been conducted for HFC-134a, HCFC-123, HCFC-22, CFC-12 and CFC-11 in a multi-fluid loop. The test pressures are up to 2 MPa (corresponding to water equivalent pressure of 7–10 MPa) and mass fluxes from 1000 to 4000 kg m −2 s −1. The heated length is adjustable from 0.5 to 1 m, the I.D. of the tubular test section is 4.2 mm, and the critical quality varies from 0.07 to 0.6. The parametric trends of test results are examined and the test results show good agreement with existing CHF prediction methods. It also provides confirmation that CFC alternatives such as HFC-134a, HCFC-123 and HCFC-22 can be used with confidence as reliable CHF modelling fluids.

Critical heat flux in horizontal annular channels with variable eccentricity

Measurements of critical heat flux (CHF) were performed for a horizontal annular channel (diameters 11.2 and 54 mm, approx. 1m heated length) in which the eccentricity in the vertical direction of the inner heater could be varied. Measurements for low mass fluxes (up to 1600 kg/m 2/s) using freon-12 at a pressure of approx. 10.3 bar and saturation inlet conditions, show a strong influence of eccentricity on CHF; this influence is discussed on a basis of the flow pattern under which CHF occurs.

Critical heat flux and droplet entrainment rate in boiling of falling liquid films

Measurements of critical heat flux and rate of droplet entrainment due to bubble bursting are made with boiling liquid films flowing downwards on the outside surface of a uniformly heated vertical tube. The critical condition occurs first at the exit end of the heating section. The critical heat flux shows three types of characteristics by increasing the liquid film flow rate at the exit end of the heating section. The data are expressed in terms of the film flow rate and the film Weber number at the exit end. The relationship between the droplet entrainment rate and the evaporation rate of the film is also discussed.

Critical Heat Flux Measurements in a 16-Rod Simulation of a BWR Fuel Assembly

Critical heat flux data were obtained for forced flow boiling in a 16-rod test section arranged in a square array. The tests were performed at 1000 psia and used a radial power distribution which represented the region about the hot corner in a BWR fuel assembly. The results are lower than data obtained in a 9-rod square array, having a uniform power distribution, based on the average bundle exit quality. These two sets of data are in fair agreement when compared on the basis of the highest subchannel exit quality. Comparisons of different sets of data show the effects of different rod spacers and bundle misalignment on the critical heat flux.

Closure to “Discussions of ‘Critical Heat Flux Measurements in a 16-Rod Simulation of a BWR Fuel Assembly’” (1969, ASME J. Heat Transfer, 91, pp. 361–362)

Closure to “Discussions of ‘Critical Heat Flux Measurements in a 16-Rod Simulation of a BWR Fuel Assembly’” (1969, ASME J. Heat Transfer, 91, pp. 361–362)