Subcooled Conditions Research Articles

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/De, 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

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/De, 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

Micro-Bubble Emission Boiling from Horizontal and Vertical Surfaces to Subcooled Parallel Flow Water.

Heat removal of more than 10MW/m2 in heat flux has been required in high heat generation equipments in nuclear fusion reactor. In some condition of water subcooling and velocity, there appears an extraordinary high heat flux boiling in transition boiling region. This boiling regime is called micro-bubble emission boiling (MEB) because a lot of micro-bubbles are spouted from the heat transfer surface accompanying a huge sound. This study is aimed to obtain heat transfer performance of MEB in horizontal and vertical heated surfaces to parallel flow of subcooled water, comparing with CHF of this system. Three types of MEB with different heat transfer performance and bubble behavior are observed according to the flow velocity and liquid subcooling.

A Generalized Performance Prediction Method for Adiabatic Capillary Tubes

Generalized dimensionless correlations were developed to predict refrigerant mass flow rate in an adiabatic capillary tube, for both subcooled and quality inlet conditions. Dimensionless parameters were created using the Buckingham Pi Theorem. These parameters were based on the effects of tube geometry, inlet conditions, and fluid properties. The correlations were based on experimental performance data with refrigerants R-134a, R-22, and R-410A that encompass an extensive range of operating conditions, including: condensing temperatures from 79 °F to 127°F (26°C to 53°C), capillary tube inner diameters from 0.026 to 0.090 in (0.66 to 2.29 mm), capillary tube lengths from 20 to 200 in (508 to 5080 mm), and inlet conditions from 30°F (16.7°C) sub-cooled to 35% quality. Corresponding to this range of refrigerants, capillary tube geometry, and operating conditions, the experimental mass flow rate range was between 3 and 375 lbm/h (1.4 to 170 kg/h). Independent assessments of both correlations were made by comparing predictions to measured performance with R-152a. These data fell within the ±6% and ±13% prediction intervals for the subcooled inlet and quality inlet correlations, respectively. The accuracy of the subcooled inlet correlation was further assessed by comparing predicted performance to measured data reported by other investigators with refrigerants R-12, R-134a, and R-22. It was shown that the subcooled inlet correlation predicted the bulk of these flow data to within ±10%. However, when compared to reported data with R-600a (isobutane), however, the subcooled inlet correlation over-predicted mass flow rate by 0 to 20%.

Nonlinear dynamics of a nuclear-coupled boiling channel with forced flows

The nonlinear dynamics of a nuclear-coupled boiling channel with forced flows was explored on the basis of the Galerkin nodal approximation method for the channel fluid flow and point kinetics for the neutron field dynamics. The marginal stability boundary (MSB) for a Freon boiling channel predicted by the model agreed well with experimental data from previous available literature, thereby verifying the model accuracy. A strip of limit cycle oscillation on the unstable side of the boundary was found to be system dependent. The marginal stability boundaries for boiling water channels with/without nuclear-coupled effects were also obtained. Moreover, the characteristics of limit cycle oscillations on the boundary were investigated. Routes from limit cycle oscillation to chaotic oscillation were identified under high inlet subcooling conditions. The strange attractor found in this study was characterized by a correlation dimension of 1.69±0.01.

FCI experiments in the corium/water system

The KROTOS fuel coolant interaction (FCI) tests are aimed at providing benchmark data to examine the effect of fuel/coolant initial conditions and mixing on explosion energetics. Experiments, fundamental in nature, are performed in well-controlled geometries and are complementary to the FARO large scale tests. Recently, a test series was performed using 3 kg of prototypical corium (80 w/o UO 2, 20 w/o ZrO 2) which was poured into a water column of ≤1.25 m in height (95 and 200 mm in diameter) under 0.1 MPa ambient pressure. Four tests were performed in the test section of 95 mm in diameter (ID) with different subcooling levels (10–80 K) and with and without an external trigger. Additionally, one test has been performed with a test section of 200 mm in diameter (ID) and with an external trigger. No spontaneous or triggered energetic FCIs (steam explosions) were observed in these corium tests. This is in sharp contrast with the steam explosions observed in the previously reported alumina (Al 2O 3) test series which had the same initial conditions of ambient pressure and subcooling. The post-test analysis of the corium experiments indicated that strong vaporisation at the melt/water contact led to a partial expulsion of the melt from the test section into the pressure vessel. In order to avoid this and to obtain a good penetration and premixing of the corium melt, an additional test was performed with a larger diameter test section. In all the corium tests an efficient quenching process (0.8–1.0 MW kg-melt −1) with total fuel fragmentation (mass mean diameter 1.4–2.5 mm) was observed. Results from alumina tests under the same initial conditions are also given to highlight the differences in behaviour between corium and alumina melts during the melt/water mixing.

Critical Heat Flux in a Vertical Annulus under Low Upward Flow and near Atmospheric Pressure

As future boiling water reactors (BWR), concepts of evolutional ABWR (ABWR-IER) and natural circulation BWR (JSBWR) have been investigated in order to reduce their construction cost and simplify their maintenance and inspection procedures. One of the promised features of the design of the evolutional ABWR is to reduce the number of internal pumps and to remove the Motor Generation (MG) sets. These design changes may induce boiling transition in the fuel rods of reactor core during a pump trip transient due to the more rapid flow coastdown characteristics than these of the present design. In addition, the understanding of critical heat flux (CHF) is one important subject to grasp safety margin during the start-up for the natural circulation BWR and to establish the rational start-up procedure in which thermo-hydraulic instabilities can be suppressed.The present study is to clarify CHF characteristics under low velocity conditions. CHF measurements were conducted in a vertical upward annulus channel composed of an inner heated rod and an outer tube made of glass. CHF data were obtained repeatedly under the condition of stable inlet flow to examine statistically their reproducibility. The flow regime was investigated from flow observation and measurement of differential pressure fluctuation. The CHF data are correlated with the flow regime transition. It was clear from the obtained flow pattern and the CHF data that the CHF behavior could be classified into specified regions by the mass flux and inlet subcooling conditions. A CHF correlation was developed and agreed with other researchers' data within acceptable error.

Experimental study on flashing concerned instability in a natural circulation system at nuclear heating reactor conditions / Experimentelle Studie zu Instabilitäten bei Siedeverzug in einem nuklearen Heizreaktor mit Naturumlauf

The experiment was performed on the test loop (HRTL-5), which simulates the geometry and system design of the 5 MW nuclear heating reactor. The flow behavior for a wide range of inlet subcoolings, in which the flow experience from single to two phase, is described in a natural circulation system at low pressure (p = 0.1, 0.24 MPa). Several kinds of flow instability are investigated, including subcooled boiling instability, subcooled boiling induced flashing instability, pure flashing instability as well as flashing coupled density wave instability and high frequency flow oscillation. The mechanism of flashing and flashing with regard to flow instability, which has never been studied well in this field, is particularly interpreted. The experimental results show that, firstly, for a low system pressure natural circulation the two phase flow is unstable in most inlet subcooling conditions, the two phase stable flow can only be achieved with very low inlet subcoolings; secondly, at high inlet subcoolings the flow instability is dominated by subcooled boiling in the heated section, and at intermediate inlet subcoolings it is dominated by void flashing in the adiabatic long riser; thirdly, in two phase stable flow region the condition for boiling out of the core, namely, single phase flow in the heated section, two phase flow in the riser due to vapor flashing, can be realized. The experimental results are very important for the design and accident analysis of the vessel and swimming pool type natural circulation nuclear heating reactor.

Evaluation of Subcooled Critical Heat Flux Correlations Using the PU-BTPFL CHF Database for Vertical Upflow of Water in a Uniformly Heated Round Tube

A simple methodology for assessing the predictive ability of critical heat flux (CHF) correlations applicable to subcooled flow boiling in a uniformly heated vertical tube is developed. Popular correlations published in handbooks and review articles as well as the most recent correlations are analyzed with the PU-BTPFL CHF database, which contains 29 718 CHF data points. This database is the largest collection of CHF data (vertical upflow of water in a uniformly heated round tube) ever cited in the world literature. The parametric ranges of the CHF database are diameters from 0.3 to 45 mm, length-to-diameter ratios from 2 to 2484, mass velocities from 0.01 × 103 to 138 × 103 kg/m2·s, pressures from 1 to 223 bars, inlet subcoolings from 0 to 347°C, inlet qualities from −2.63 to 0.00, outlet subcoolings from 0 to 305°C, outlet qualities from −2.13 to 1.00, and CHFs from 0.05 × 106 to 276 × 106 W/m2. The database contains 4357 data points having a subcooled outlet condition at CHF. A correlation published elsewhere is the most accurate in both low- and high-mass velocity regions, having been developed with a larger database than most correlations. In general, CHF correlations developed from data covering a limited range of flow conditions cannot be extended to other flow conditions without much uncertainty.

Evaluation of the Critical Heat Flux Under Conditions of High Subcooling and High Velocity in Small Diameter Channels.

The critical heat flux (CHF) under conditions of high subcooling and high velocity in small diameter channels was systematically evaluated to obtain a reliable CHF correlation which can be used to predict CHFs higher than 50 MW/m2 in the design of high performance nuclear research reactors, the accelerator target and the first wall and diverter of the fusion reactor etc.. The major test conditions of the existing CHF experiments investigated in this study were a channel diameter or gap of 0.3 to 4mm, a channel length-to-diameter ratio of 15 to 97, a pressure of 0.1 to 7.1 MPa, an inlet water subcooling of 13 to 210°C and a water mass flux of 124 to 90000kg/(m2·s), resulting in a CHF of 4.2 to 224MW/m2. The existing experimental data were compared with the analytical CHF solution which was derived by the author for the channel flow under high subcooling and high velocity of water at atmospheric pressure. As a result, the effect of the channel diameter on CHF was clarified quantitatively and a correlation of CHF was newly proposed, which can be used to predict the existing CHF data within an error of about ±35 percent, reflecting the effect of the pressure properly.

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 were, for the first time, systematically and quantitatively investigated to provide a CHF correlation which could properly predict the effect of channel length, especially when the channel length-to-channel diameter ratio L/D is less than about 20. Major test conditions of existing CHF experiments investigated in this study were 1 to 4 mm of channel diameter, 1 to 25 of L/D, 0.1 to 1.2 MPa of pressure, 34 to 117°C of inlet water subcooling and 500 to 40 700 kg/(m2·s) of mass flux in circular channels, and 3 to 20 mm of gap size, 6 to 40 of L/De, 0.1 to 3.1 MPa of pressure, 4 to 166°C of inlet water subcooling and 940 to 27000 kg/(m2·s) of 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 trnasfer of the 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 could predict CHF within a ±35 percent error margin.

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

96/06501 Experiments with replacement refrigerants over a wide range of applications

A capillary tube is a common expansion device widely used in small-scale refrigeration and air-conditioning systems. A generalized correlation of refrigerant mass flow rate through adiabatic capillary tubes covering both subcooled and two-phase inlet conditions is expected for multiple purposes. Based on the homogeneous equilibrium flow model, a new group of dimensionless parameters has been proposed. To express the nonlinear relationship between the mass flow rate and the associated parameters, the multi-layer perceptron neural network is employed as a universal function approximator. Simulated data from a validated homogeneous equilibrium model are used for the neural network training and testing. A 5-6-1 network trained with the simulated data of R600a and R407C shows good generality in predicting the simulated data of R12, R22, R134a, R290, R410A, and R404A. Also, the deviations between the trained neural network and the experimental data from the open literature fall into ±10%.

Study on Critical Heat Flux in Rectangular Channels Heated From One or Both Sides at Pressures Ranging From 0.1 to 14 MPa

In this study, a quantitative analysis of critical heat flux (CHF) in rectangular heated channels was carried out based on new flow models and the analytical results were compared with existing experimental results at pressures of about 0.1 to 14 MPa with a water mass flux of 3.9 to 28,000 kg/m2s and inlet water subcooling ranging from 0 to 328 K. The flow models proposed for CHF were a completely separated two-phase flow model with a macroliquid sublayer under conditions of comparatively low velocity and zero water subcooling at outlet of the channel and a subcooled boiling flow model with a macroliquid sublayer under the conditions of high water subcooling and high velocity, respectively. It could be shown that the analytical CHF results gave good predictions for over 800 existing experimental results, identifying the effects of predominant parameters as regards CHF.

Chaotic oscillations in a low pressure two-phase natural circulation loop under low power and high inlet subcooling conditions

The oscillation characteristics of a low pressure two-phase natural circulation loop have been investigated experimentally in this study. Experimental results indicate that the characteristics of the thermal hydraulic oscillations can be periodic, with 2–5 fundamental frequencies, or chaotic, depending on the heating power and inlet subcooling. The number of fundamental frequencies of oscillation increases if the inlet subcooling is increased at a given heating power or the heating power is decreased at a given inlet subcooling; chaotic oscillations appear if the inlet subcooling is further increased and/or the heating power is further decreased. A map of the oscillation characteristics is thus established. The change in oscillation characteristics is evident from the time evolution and power spectrum of a thermal hydraulic parameter and the phase portraits of two thermal hydraulic parameters. These results reveal that a strange attractor exists in a low pressure two-phase natural circulation loop with low power and very high inlet subcooling.

Estimation of Kapitza conductance effect on steady and transient boiling heat transfer in He I based on Kapitza conductance results in He II

Kapitza conductance in steady and transient boiling heat transfer on a horizontal 0.2-mm diameter test wire was estimated based on the measured Kapitza conductance value for the same surface in He II. The He II experiments performed are for liquid temperatures ranging from 1.8 to 2.1 K under atmospheric pressure, and the He I experiments of the steady and transient heat transfer caused by exponential heat inputs are for the periods ranging from 0.2 ms to 10s under saturated conditions at liquid temperatures ranging from 2.2 to 4.2 K, and subcooled conditions for the subcooling of 1.0 and 2.0 K at atmospheric pressure. The Kapitza conductance in He I is higher for higher saturation temperatures but little dependent upon heat flux, liquid subcooling, and heat input increasing rate. The temperature drop due to the Kapitza conductance for the wall superheat becomes more significant with the increase in heat flux and in saturation temperature. It was confirmed that the Kapitza conductance in He I can be estimated correctly based on the Kapitza conductance value for the same surface in He II.

Analytical Study on Characteristics of Critical Heat Flux under High Subcooling and High Velocity Conditions.

A quantitative analysis of critical heat flux (CHF) under high mass flux with high subcooling was successfully carried out by applying a new transition region model for a thin liquid sublayer on the heated walls to the existing model of vapor blanket over the thin liquid sublayer. The CHF correlation proposed in this study could predict well experimental data obtained using circular tubes of 2.5 to 3 mm in diameter, 100 to 290 mm in length, 0.34 to 2.6 MPa and 13 to 210°C in inlet subcooling, and rectangular channels of 3.5 mm in gap, 150 mm in length, 0.5 to 3.1 MPa and 13 to 166°C in inlet subcooling, revealing a unique feature that the critical heat flux is proportional to around G0.8, where G is the mass flux, at high mass flux of the dimensionless mass flux G* ranging from 350 to 4700 under the subcooled condition at the exit 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

Two-dimensional impinging jet cooling of high heat flux surfaces in magnetic confinement fusion reactors

The divertor surface of a magnetic confinement fusion reactor is exposed to strong radiation heating by high flux charged particles. According to the standard design for the ITER, the heat flux on the divertor surface averages 15 MW m −2 or more. In this study, cooling by a two-dimensional impinging jet flow is proposed to cool this surface. For an impinging jet flow on a flat heated surface, a high critical heat flux (CHF) is obtained only in the limited surface region where the jet flow hits directly. Outside this region, the CHF decreases abruptly with distance from the center. The main reason is that the pressure decreases abruptly away from the center region and the liquid flow is spread away from the heated surface region by the strong boiling. To overcome these difficulties, we propose that the impinging jet is applied to a heat transfer wall with a concave surface. In this study, the CHFs and the nucleate boiling curves for two-dimensional impinging jet cooling were first obtained as a function of the distance from the center using a thin copper foil heater designed on the plastic sheet. Experiments were done under various conditions of liquid subcooling, flow velocity and surface curvature. Empirical correlations for the CHF including these parameters were obtained. It is clear that impinging jet cooling of the curved surface is useful to keep the CHF in the downstream region high. Finally, application of the two-dimensional jet to the cooling of a fusion divertor surface is assessed. When the jet velocity is equal to 14.6 m −1 , and the liquid subcooling is 80 K, a two-dimensional jet is able to cool a curved surface area 50 mm wide with an average heat flux of 30 MW m −2 .

A dielectric surface coating technique to enhance boiling heat transfer from high power microelectronics

Two benign methods of generating surface microstructures, spraying and painting, provide pool boiling heat transfer enhancement. The methods do not require the target surface to be exposed to high stress environments, making them applicable to electronic component surfaces. The painted surface microstructures are applied to a rectangular, horizontally oriented surface, and provide up to a 85% reduction in incipient superheat. Between a 70 and 80% reduction in nucleate boiling superheats, and as much as a 100% increase in the maximum heat flux (critical heat flux-CHF), beyond that of the nontreated reference surface. The surface microstructures are also applied to a silicon test chip and tested at saturated and sub-cooled (45/spl deg/C) conditions using FC-72. At sub-cooled conditions, heat dissipation rates of 100 W/cm/sup 2/ were provided at junction temperatures of 85/spl deg/C, and the highest CHF observed was 159 W/cm/sup 2/, 224% higher than that from the untreated chip surface at saturated conditions. >