Departure From Nucleate Boiling Heat Flux Research Articles

DNB heat flux on inner side of a vertical pipe in forced flow of liquid hydrogen and liquid nitrogen

Heat transfer from inner side of a heated vertical pipe to liquid hydrogen flowing upward was measured at the pressures of 0.4, 0.7 and 1.1 MPa for wide ranges of flow rate and liquid temperature. Nine test heaters with different inner diameters of 3, 4, 6 and 9 mm and the lengths of 50, 100, 150, 200, 250 and 300 mm were used. The DNB (departure from nucleate boiling) heat fluxes in forced flow of liquid hydrogen were measured for various subcoolings and flow velocities at pressures of 0.4, 0.7 and 1.1 MPa. Effect of L/d (ratio of heater length to diameter) was clarified for the range of L/d⩽50. A new correlation of DNB heat flux was presented based on a simple model and the experimental data. Similar experiments were performed for liquid nitrogen at pressures of 0.5 MPa and 1.0 MPa by using the same experimental system and some of the test heaters. It was confirmed that the new correlation can describe not only the hydrogen data, but also the data of liquid nitrogen.

DNB heat flux in forced convection of liquid hydrogen for a wire set in central axis of vertically mounted flow channel

Liquid hydrogen has excellent physical properties, high latent heat and low viscosity of liquid, as a coolant for superconductors like MgB2. The knowledge of Departure from Nucleate Boiling (DNB) heat flux of liquid hydrogen is necessary for designing and cooling analysis of high critical temperature superconducting devices. In this paper, DNB heat fluxes of liquid hydrogen were measured under saturated and subcooled conditions at absolute pressures of 400, 700 and 1100 kPa for various flow velocities. Two wire test heaters made by Pt-Co alloy with the length of 200 mm and the diameter of 0.7 mm were used. And these round heaters were set in central axis of a flow channel made of Fiber Reinforced Plastic (FRP) with inner diameters of 8 mm and 12 mm. These test bodies were vertically mounted and liquid hydrogen flowed upward through the channel. From these experimental values, the correlations of DNB heat flux under saturated and subcooled conditions are presented in this paper.

Equation of DNB Heat Flux for Upward Forced Flow of Cryogenic Liquids

Abstract Knowledge of departure from nucleate boiling (DNB) heat flux is important for design of superconducting systems cooled by cryogenic liquids. We have already presented the equation of DNB heat flux that can describe the experimental data of liquid hydrogen. To see the applicability of the equation to other cryogenic liquids, similar heat transfer tests in forced flow of liquid nitrogen are performed for wide ranges of conditions in this work. It was confirmed that the DNB heat flux equation derived by the authors can express not only the data for liquid hydrogen but also those for liquid nitrogen.

Forced Convection Heat Transfer of Liquid Hydrogen Through a 200-mm Long Heated Tube

Abstract The heat transfer from the inner side of a vertically- mounted heated tube with a length of 200.0 mm and a diameter of 6.0 mm to a forced flow of liquid hydrogen was measured for wide ranges of flow rate and liquid temperature. The non-boiling heat transfer coefficients agreed well with the Dittus -Boelter equation. The heat fluxes at departure from nucleate boiling (DNB) were higher for higher flow velocities and greater subcooling. The effect of the tube length on the DNB heat flux was clarified through comparison with our previous data. It was confirmed that the experimental data agreed well with the authors’ DNB correlation.

Forced convection heat transfer of subcooled liquid nitrogen in a vertical tube

Experimental research on forced convection heat transfer of subcooled liquid nitrogen ranging from the pressures of 0.3 MPa to its supercritical pressure is carried out for wide ranges of inlet temperature and flow velocity. A stainless steel tube heater with the inner diameter of 5.4 mm and the length of 100 mm is mounted vertically. The heat transfer coefficients in non-boiling region and the DNB (departure from nucleate boiling) heat fluxes are higher for higher flow velocity and higher subcooling. The trend of the heat transfer coefficients in the non-boiling region agree with those by the Dittus-Boelter correlation, although they are unaffected by the flow velocity for Re < 8000. The lowest limits agree with those obtained for no forced flow. The correlation of DNB heat flux that can describe the experimental data is presented. On the other hand, forced convection heat transfer of supercritical nitrogen deteriorates when the heated surface temperature, Tw, exceeds its pseudo-critical temperature, Tc', although they are similar to those under subcritical pressure for Tw < Tc'. The heat transfer characteristics of supercritical nitrogen can be predicted by authors' correlation.

Forced flow boiling heat transfer of liquid hydrogen for superconductor cooling

Heat transfer from inner side of a heated vertical pipe to liquid hydrogen flowing upward was first measured at the pressure of 0.7 MPa for wide ranges of flow rates and liquid temperatures. The heat transfer coefficients in non-boiling regime for each flow velocity were well in agreement with the Dittus–Boelter equation. The heat fluxes at the inception of boiling and the departure from nucleate boiling (DNB) heat fluxes are higher for higher flow velocity and subcooling. It was found that the trend of dependence of the DNB heat flux on flow velocity was expressed by the correlation derived by Hata et al. based on their data for subcooled flow boiling of water, although it has different propensity to subcooling.

DNB experiments for high-conversion PWR core design

It is very important to clarify the departure from nucleate boiling (DNB) performance of core fuel assemblies for the high conversion pressurized water reactors (PWR) [1,2]. To investigate this, DNB experiments were performed in tight lattice rod bundles, using the model fluid Freon 12 and water under the actual operating conditions. In addition, DNB heat flux measurements in an annular-flow channel were carried out for the design of fertile rods, which are installed in thimble tubes.

Experimental study of differences in DNB heat flux between upflow and downflow in vertical rectangular channel.

Experiments were carried out on DNB (Departure from Nucleate Boiling) heat flux for both upflow and downflow in a rectangular vertical channel simulating a subchannel in the fuel element of the research reactor JRR-3, which is scheduled to be upgraded at 20MWt with 20% low enriched uranium (LEU) fuel. The experiments were carried out for the conditions of pressure and velocity which are important for the safety design of the JRR-3. With the investigation of the data of the present experiment along with already existing data for both rectangular channels and other channels, a scheme of DNB heat flux correlations was obtained for upflow and downflow. With the all available data, the errors of the correlations adopted in this scheme were so evaluated as to utilize these results in the safety analysis of the JRR-3. This scheme includes a new correlation of DNB heat flux for upflow and the identification of a region of high coolant flow rate where no remarkable differences in the DNB heat flux are observed bet...

Rod Bowed to Contact Departure from Nucleate Boiling Tests in Coldwall Thimble Cell Geometry

Departure from nucleate boiling (DNB) tests were conducted to measure the effect of a rod bowed to contact in a coldwall thimble cell of a pressurized water reactor. The tests were conducted using electrically heated 4×4 rod bundles with 4,270mm heated length and with axially and radially non-uniform heat flux, locating mixing vane grids on a 660 mm spacing. One of central heater rods was bowed into contact at the topmost grid span where most of DNB were observed in the unbowed geometry test. The tests were resulted in no effect of a rod bowed to contact at low pressure (105 and 127 ata) but considerable effect at high pressure (148 and 169 ata). The reduction of DNB heat flux was given by a function of rod average heat flux and pressure.

DNB in liquid metal heated forced convection boiling

An experimental program was performed to investigate departure from nucleate boiling (DNB) in a flow boiling system heated by a liquid metal flow. The boiling fluid, freon 12, was circulated vertically upward and was heated by countercurrent flowing mercury. Magnitudes and trends of the heat flux and quality at DNB were compared to data obtained on a similar experimental facility employing uniform electrical heating of the test section wall. The results show that at a given quality, the magnitude of the DNB heat flux is larger in the case of uniform electrical heating. This result is not predicted by a DNB correlation based on data obtained from electrically heated test sections employing uniform and non-uniform axial heat flux distributions. It is demonstrated that the DNB phenomenon in a temperature controlled system (liquid metal heated) may not be well predicted from correlations based on data (uniformly or nonuniformly heated) obtained from heat flux controlled systems (electrically heated).

An investigation of departure from nucleate boiling (DNB) in a crossed-rod matrix with normal flow of freon-113 coolant

While the boundary layer behavior dictates the heat transfer of a single-phase flow, the bubble layer dominates the boiling heat transfer of a low void fraction two-phase flow. The bubble layer is a highly viscous layer of crowded bubbles flowing parallel to the heating surface. In a subcooled or low quality boiling flow, the bubble layer locates between the main liquid stream and the heated wall. It shields the wall from the incoming cooling liquid so that the wall can be overheated and reach the heat flux density causing Departure from Nucleate Boiling (DNB). Increase in the DNB heat flux of the rod bundle in a PWR core can be achieved by breaking the bubble layer. One way to break the bubble layer is to introduce the coolant flow normal to the fuel rods. Normal flow may induce vibration of the rod banks. However, flow-induced vibration is absent in a crossed-rod matrix when two adjacent banks of rods are in contact at a 90 degree angle. However, this may cause concern to the reactor designer, because the rod-contact point produces a hot spot which might cause a premature DNB. This paper reports the DNB heat fluxes measured in freon flow normal to a crossed-rod matrix having all rods heated. The result is that the DNB heat flux at the rod-contact point is much higher than that in flow parallel to the rod bundles.

Two Regimes of Burnout (DNB) Correlated with Steam Energy Flow for Uniformly-Heated Channels

The scatter in DNB (departure from nucleate boiling) correlations is minimized if the DNB heat flux is plotted vs G(h--h/sub f/), the rate of flow of enthalpy of vaporization across unit flow area or steam energy flow. Two distinct regimes exist in this kind of graph, one associated with thermal instability (DNB-1) and the other with hydrodynamic instability (DNE-2). The characteristics of each regime are discussed in some detail. Most H/sub 2/O- and D/sub 2/O-cooled reactors operate below the DNB-1 regime, and thus burnout by hydrodynamic instability is much more likely than by thermal instability. (D.L.C.)