Exit Quality Research Articles

The Critical Heat Flux Condition With Water in a Uniformly Heated Microtube

The critical heat flux (CHF) condition needs to be well understood for designing miniature devices involving two-phase flow. Experiments were performed to determine the CHF condition for a single stainless steel tube having an inside diameter of 0.427mm subjected to uniform heat flux boundary conditions. The effects of mass flux, pressure, and exit quality on the CHF were investigated. The experimental results show that the CHF increases with an increase in mass flux and exit pressure. For all exit pressures, the CHF decreased with an increase in quality in the subcooled region, but with a further increase in quality (near zero quality and above), the CHF was found to have an increasing trend with quality (up to about 25% quality). CHF values in this region were much higher than those in the subcooled region. This suggests that even at very low qualities, the void fraction becomes appreciable, which results in an increase in the average velocity, thereby increasing the CHF limit.

High-Speed Visualization of Two-Phase Flow in a Micro-Scale Pin-Fin Heat Exchanger

Flow regimes and bubble growth are observed in a pin-fin micro-scale heat exchanger with R-11 as the working fluid. The heat exchanger is machined in silicon and derived from a DNA micro-array consisting of 150 μm-square fins separated by 50 μm-square passages. The fins are staggered and oriented 45 degrees to the flow direction such that approximately 750 channel intersections occur within the volume of the exchanger. The purpose of the study is to determine if this multiply-connected geometry produces the flow blockage, reversal, and other instabilities observed in single and parallel micro-channel configurations. The upper surface of the exchanger is a glass plate that provides optical access. High-speed digital photography and microscope optics are used to obtain real-time images of the flow at a framing rate of 5 kHz. The lower surface is electrically heated and instrumented with a heat flux gage. Inlet and outlet temperatures and pressures, heater and wall temperatures, and volumetric flow rate are monitored. Nucleation is observed near the entrance of the heat exchanger. In the central section, developed vapor regions are composed of broad slug-like vapor fronts immediately followed by a slowly growing bubbly flow. An annular regime dominates the downstream section of the exchanger with drop-like liquid structures appearing at the downstream edge of fins. The heat transfer coefficient decreases with exit quality as in other micro-scale exchangers; however, the flow instability present in parallel channel exchangers is not observed in this configuration.

Two-phase flow instability for boiling in a microchannel heat sink

The present study explores experimentally the two-phase flow instability in a microchannel heat sink with 15 parallel microchannels. The hydraulic diameter for each channel is 86.3 μm. Flow boiling in the present microchannel heat sink demonstrates significantly different two-phase flow patterns under stable or unstable conditions. For the stable cases bubble nucleation, slug flow and slug or annular flows appear sequentially in the flow direction. On the other hand, forward or reversed slug/annular flows appear alternatively in every channel. Moreover, the length of bubble slug may oscillate for unstable cases with reversed flow demonstrating the suppressing effect of pressure field for bubble growth. It is found that the magnitude of pressure drop oscillations may be used as an index for the appearance of reversed flow. A stability map on the plane of inlet subcooling number versus phase change number is established. A very narrow region for stable two-phase flow or mild two-phase flow oscillations is present near the line of zero exit quality.

Pressure drop studies on two-phase flow in a uniformly heated vertical tube at pressures up to the critical point

Measurements of two-phase flow pressure drop have been made during a phase-change heat transfer process with refrigerant (R-134a) as a working fluid for a wide range of pressures right up to the critical pressure. The experiments were conducted in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length over a heat flux range of 35–80 kW/m 2, mass flux range of 1200–2000 kg/m 2 s, exit quality range of 0.19–0.81 and for reduced pressures ranging from 0.24 to 1 with a fixed inlet subcooling of 3 °C. The measurements were compared with the predictions from the homogeneous flow model, a separated flow model using correlations drawn from the literature for void fraction and frictional pressure drop, and finally, using a flow pattern-based predictive method accounting specifically for bubbly, slug and annular flow regimes. It was found that the best results were obtained with the flow pattern-based approach with a mean deviation of ±20% over the entire pressure range.

Flow Boiling Heat Transfer in Microchannels

Flow boiling heat transfer to water in microchannels is experimentally investigated. The dimensions of the microchannels considered are 275×636 and 406×1063μm2. The experiments are conducted at inlet water temperatures in the range of 67–95°C and mass fluxes of 221–1283kg∕m2s. The maximum heat flux investigated in the tests is 129W∕cm2 and the maximum exit quality is 0.2. Convective boiling heat transfer coefficients are measured and compared to predictions from existing correlations for larger channels. While an existing correlation was found to provide satisfactory prediction of the heat transfer coefficient in subcooled boiling in microchannels, saturated boiling was not well predicted by the correlations for macrochannels. A new superposition model is developed to correlate the heat transfer data in the saturated boiling regime in microchannel flows. In this model, specific features of flow boiling in microchannels are incorporated while deriving analytical solutions for the convection enhancement factor and nucleate boiling suppression factor. Good agreement with the experimental measurements indicates that this model is suitable for use in analyzing boiling heat transfer in microchannel flows.

Particular characteristics of transcritical CO 2 refrigeration cycle with an ejector

The present study describes a theoretical analysis of a transcritical CO 2 ejector expansion refrigeration cycle (EERC) which uses an ejector as the main expansion device instead of an expansion valve. The system performance is strongly coupled to the ejector entrainment ratio which must produce the proper CO 2 quality at the ejector exit. If the exit quality is not correct, either the liquid will enter the compressor or the evaporator will be filled with vapor. Thus, the ejector entrainment ratio significantly influences the refrigeration effect with an optimum ratio giving the ideal system performance. For the working conditions studied in this paper, the ejector expansion system maximum cooling COP is up to 18.6% better than the internal heat exchanger cycle (IHEC) cooling COP and 22.0% better than the conventional vapor compression refrigeration cycle (VCRC) cooling COP. At the conditions for the maximum cooling COP, the ejector expansion cycle refrigeration output is 8.2% better than the internal heat exchanger cycle refrigeration output and 11.5% better than the conventional cycle refrigeration output. An exergy analysis showed that the ejector expansion cycle greatly reduces the throttling losses. The analysis was also used to study the variations of the ejector expansion cycle cooling COP for various heat rejection pressures, refrigerant temperatures at the gas cooler exit, nozzle efficiencies and diffuser efficiencies.

Thermal analysis of plate condensers in presence of flow maldistribution

Flow maldistribution in plate heat exchangers causes deterioration of both thermal and hydraulic performance. The situation becomes more complicated for two-phase flows during condensation where uneven distribution of the liquid to the channels reduces heat transfer due to high liquid flooding. The present study evaluates the thermal performance of falling film plate condensers with flow maldistribution from port to channel considering the heat transfer coefficient inside the channels as a function of channel flow rate. A generalized mathematical model has been developed to investigate the effect of maldistribution on the thermal performance as well as the exit quality of vapor. A wide range of parametric study is presented, which shows the effects of the mass flow rate ratio of cold fluid and two-phase fluid, flow configuration, number of channels and correlation for the heat transfer coefficient. The analysis presented here also suggests an improved method for heat transfer data analysis for plate condensers.

Critical Heat Flux of R-123 in Silicon-Based Microchannels

Critical heat flux (CHF) of R-123 in a silicon-based microchannel heat sink was investigated at exit pressures ranging from 227kPato520kPa. Critical heat flux data were obtained over effective heat fluxes ranging from 53W∕cm2to196W∕cm2 and mass fluxes from 291kg∕m2sto1118kg∕m2s. Flow images and high exit qualities suggest that dryout is the leading CHF mechanism. The effect of mass velocity, exit quality, and system pressure were also examined, and a new correlation is presented to represent the effect of these parameters.

Progress of the DUPIC Fuel Compatibility Analysis - II: Thermal Hydraulics

Thermal-hydraulic compatibility of the DUPIC fuel bundle with a 713-MW(electric) Canada deuterium uranium (CANDU-6) reactor was studied by using both the single-channel and subchannel analysis methods. The single-channel analysis provides the fuel channel flow rate, pressure drop, critical channel power, and the channel exit quality, which are assessed against the thermal-hydraulic design requirements of the CANDU-6 reactor. The single-channel analysis by the NUCIRC code showed that the thermal-hydraulic performance of the DUPIC fuel is not different from that of the standard CANDU fuel. Regarding the local flow characteristics, the subchannel analysis also showed that the uncertainty of the critical channel power calculation for the DUPIC fuel channel is very small. As a result, both the single- and subchannel analyses showed that the key thermal-hydraulic parameters of the DUPIC fuel channel do not deteriorate compared with the standard CANDU fuel channel.

Critical Heat Flux Experiments on the Reactor Vessel Wall Using 2-D Slice Test Section

The critical heat flux (CHF) on the reactor vessel outer wall was measured using the two-dimensional slice test section. The radius and the channel area of the test section were 2.5 m and 10 cm × 15 cm, respectively. The flow channel area and the heater width were smaller than those of the ULPU experiments, but the radius was greater than that of the ULPU. The CHF data under the inlet subcooling of 2 to 25°C and the mass flux 0 to 300 kg/m2·s had been acquired. The measured CHF value was generally slightly lower than that of the ULPU. The difference possibly comes from the difference of the test section material and the thickness. However, the general trend of CHF according to the mass flux was similar with that of the ULPU. The experimental CHF data were compared with the predicted values by SULTAN correlation. The SULTAN correlation predicted well this study’s data only for the mass flux higher than 200 kg/m2·s, and for the exit quality lower than 0.05. The local condition-based correlation was developed, and it showed good prediction capability for broad quality (-0.01 to 0.5) and mass flux (<300 kg/m2·s) conditions with a root-mean-square error of 2.4%. There were increases in the CHF with trisodium phosphate-added water.

Studies on critical heat flux in flow boiling at near critical pressures

Experimental studies on critical heat flux (CHF) have been conducted in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length at different reduced pressures ranging from 0.24 to 0.99 with R-134a as the working fluid. The onset of CHF was determined by the sudden rise in the wall temperature of the electrically heated tube. Experiments were performed over a wide range of parameters: mass flux values from 200 to 2000 kg/m 2 s, pressure from 10 to 39.7 bars and heat flux from 2 to 80 kW/m 2 and exit quality from 0.17 to 0.94. The results show considerably lower critical heat flux at high pressures. Well known CHF prediction methods, such as the look-up table and correlations of earlier workers show poor agreement at high pressures. A new correlation has been proposed to estimate the CHF in uniformly heated vertical tubes up to the critical pressure and over a wide range of parameters.

Implementation of Microchannel Evaporator for High-Heat-Flux Refrigeration Cooling Applications

While most recently electronic cooling studies have been focused on removing the heat from high-power-density devices, the present study also explores means of greatly decreasing the device operating temperature. This is achieved by incorporating a microchannel heat sink as an evaporator in an R134a refrigeration loop. This system is capable of maintaining device temperatures below 55°C while dissipating in excess of 100W∕cm2. It is shown that while higher heat transfer coefficients are possible with greater mass velocities, those conditions are typically associated with wet compression corresponding to evaporator exit quality below unity and liquid entrainment at the compressor inlet. Wet compression compromises compressor performance and reliability as well as refrigeration cycle efficiency and therefore must be minimized by maintaining only slightly superheated conditions at the compressor inlet, or using a wet-compression-tolerant compressor. A parametric study of the effects of channel geometry on heat sink performance points to channels with small width and high aspect ratio as yielding superior thermal performance corresponding to only a modest penalty in pressure drop.

Density Wave Instability of Sodium Boiling Two-phase Flow in a Vertical Annulus at Low Pressure

Experiments of density wave instability in a sodium boiling two-phase flow in an annulus were carried out with the parameters of heat flux from 80 to 976kW/m2, inlet subcooling from 25.6 to 226.8°C, mass flow rate from 7.92 to 68.9 kg/h, and system pressure from 2,600 Pa to 0.06 MPa. It was found that the density wave instability occurred in the case of low exit quality, and the oscillation of flow rate was so large that the flow would be reversal. The lower inlet temperature, the higher system pressure and the larger mass flow rate could result in a more stable boiling two-phase flow. The oscillation period of the instability increased with the system pressure and the inlet subcooling, but it decreased with the mass flow rate. A correlation for the onset condition of the density wave instability was obtained from the experimental data.

Two-phase pressure drop of water during flow boiling in a vertical narrow channel

Two-phase pressure drop during flow boiling of water in a vertical channel with cross-section 2 mm by 1 mm was investigated experimentally. The experiments were performed for the conditions of heat flux from 25 to 105 kW/m 2, mass flux from 57 to 211 kg/m 2s and exit quality from 9% to 30%, including the influence of inlet compressibility. The two-phase pressure distribution along the channel was recorded at 200 Hz sampling frequency. The results were compared with four correlations. The Lockhart–Martinelli correlation for pressure drop in large tubes with modified C value has been shown to be still applicable with uncertainty of 35%. The homogeneous model had an uncertainty of 50%; the Tran and Chisholm methods over-estimated the experimental data by 100% and 300% respectively. It is shown that none of these correlations can predict published pressure drop data for water during flow boiling in a 50 by 70 micron channel.

Density Wave Instability of Sodium Boiling Two-phase Flow in a Vertical Annulus at Low Pressure

Experiments of density wave instability in a sodium boiling two-phase flow in an annulus were carried out with the parameters of heat flux from 80 to 976kW/m2, inlet subcooling from 25.6 to 226.8°C, mass flow rate from 7.92 to 68.9 kg/h, and system pressure from 2,600 Pa to 0.06 MPa. It was found that the density wave instability occurred in the case of low exit quality, and the oscillation of flow rate was so large that the flow would be reversal. The lower inlet temperature, the higher system pressure and the larger mass flow rate could result in a more stable boiling two-phase flow. The oscillation period of the instability increased with the system pressure and the inlet subcooling, but it decreased with the mass flow rate. A correlation for the onset condition of the density wave instability was obtained from the experimental data.

Study on the stability behaviour of a natural circulation pressure tube type boiling water reactor

The stability behaviour of a natural circulation pressure tube type boiling water reactor (BWR) has been investigated analytically. The analytical model considers homogeneous two-phase flow, a point kinetics model for the neutron dynamics and a lumped heat transfer model for the fuel dynamics. The results indicate that both Type I and Type II density-wave instabilities can occur in the reactor in both in-phase and out-of-phase mode of oscillations in the boiling channels of the reactor. The delayed neutrons were found to have strong influence on the stability of Type I and Type II density-wave instabilities. Also, the stability of the reactor is found to increase with increase in negative void reactivity coefficient unlike that observed previously in vessel type BWRs. Decay ratio map was predicted considering the effects of channel power, channel inlet subcooling, feed water temperature and channel exit quality, which are useful for the design of the reactor.

A Study on Thermo-Hydraulic Instability of Boiling Natural Circulation Loop with a Chimney. 4th Report. An Analytical Consideration of the Stability and Thermo-Hydraulic Characteristics in the Chimney in High Pressure.

Thermo-hydraulic instabilities of a boiling natural circulation loop with a chimney under high pressure were investigated using linear stability analysis. Drift-flux model was used for two-phase flow model. The instability regions as well as the thermo-hydraulic characteristics in the chimney such as wavy feature were examined, which were compared with the characteristics in low pressure. Instability could occur when exit quality was relatively low, which was the same manner as the characteristics in low pressure. In high-pressure, void was generated near channel exit, and void wave propagated in the chimney. In low pressure, steam was generated only near the chimney exit due to gravity induced flashing, and single-phase enthalpy wave, that is, temperature wave propagated in single-phase flow region. Though flow could be very stable in the high pressure and high power condition, the decay ratio of higher mode could be larger than that of lower mode.

Theoretical and experimental study on density wave oscillation of two-phase natural circulation of low equilibrium quality

A theoretical and experimental study of density wave oscillation (DWO) in natural circulation is presented in this paper. Experiments were performed on a natural circulation test facility. The influences of mass flow rate, pressure, inlet subcooling, heat flux and exit quality on DWO were analyzed. The marginal stability boundary (MSB) of DWO was obtained. A criterion of two-phase natural circulation, which predicts the stability thresholds, was developed by lumped parameter method. It is a function of non-dimensional parameters, such as phase change number N pch, subcooling number N sub, Froude number, Fr, geometry number N l and friction number τ. The geometry number and friction number are first defined in this paper. A correlation of DWO period was also obtained, it is also a function of the above non-dimensional parameters. The results of the present criterion and period correlation were compared with those of the experimental data and references. It is shown that they agree very well.

Flow boiling heat transfer in a vertical spirally internally ribbed tube

Experiments of flow boiling heat transfer and two-phase flow frictional pressure drop in a spirally internally ribbed tube (φ22×5.5 mm) and a smooth tube (φ19×2 mm) were conducted, respectively, under the condition of 6×105 Pa (absolute atmosphere pressure). The available heated length of the test sections was 2500 mm. The mass fluxes were selected, respectively, at 410, 610 and 810 kg/m2 s. The maximum heat flux was controlled according to exit quality, which was no more than 0.3 in each test run. The experimental results in the spirally internally ribbed tube were compared with that in the smooth tube. It shows that flow boiling heat transfer coefficients in the spirally internally ribbed tube are 1.4–2 times that in the smooth tube, and the flow boiling heat transfer under the condition of smaller temperature differences can be achieved in the spirally internally ribbed tube. Also, the two-phase flow frictional pressure drop in the spirally internally ribbed tube increases a factor of 1.6–2 as compared with that in the smooth tube. The effects of mass flux and pressure on the flow boiling heat transfer were presented. The effect of diameters on flow boiling heat transfer in smooth tubes was analyzed. Based on the fits of the experimental data, correlations of flow boiling heat transfer coefficient and two-phase flow frictional factor were proposed, respectively. The mechanisms of enhanced flow boiling heat transfer in the spirally internally ribbed tube were analyzed.

Thermalhydraulic Investigation Using a Test Facility Simulating 200 MW Integral Heating Reactor

Since the middle of 1980's, a research program on heating reactor has been carried out at Institute of Nuclear Energy Technology (INET) of Tsinghua University in China, the heating reactor is a light water cooled, integral type natural circulation reactor with low system pressure and low core exit quality, it has the passive safety features and other advanced characteristics of the next generation reactors. Because of a relatively long riser and low system pressure, a small quality perturbation at the exit of the core will lead to a relatively large variation of void fraction in the riser, so will the driving force. All of the above features made it very different from the typical PWRs and BWRs. In order to guarantee the safe operation of the heating reactor, a full scale test loop HRTL-200, simulating HR-200 heating reactors in main geometry and operation parameters, was constructed and operated under the broad test conditions covering the 200 MW heating reactor operation conditions at INET. The HRTL-200 test facility, some of steady-state and two-phase flow instability test results from the test facility and the consideration how to choose the optimal design parameters for the heating reactors are shown in this paper. The range of experimentally studied parameters are: the system pressure is from 1.0 to 4.0 MPa, the largest heat flux is about 50 W/cm2, and the quality at the exit of the test section is less than 6%.