High Inlet Subcooling Research Articles

Two-phase flow instabilities during microgravity flow boiling onboard the International Space Station

This study is an elaboration on flow instabilities observed during flow boiling experiments conducted onboard the International Space Station (ISS) as part of the Flow Boiling and Condensation Experiment (FBCE). During highly subcooled flow boiling, liquid backflow into the channel that rapidly condenses vapor within the channel was observed. Experiments were conducted with an enhanced sampling frequency of 30 Hz and an extended image sequence recording duration of at least 4 seconds at 500 frames per second to further investigate the instability. Identical experiments were performed both in microgravity onboard the International Space Station (ISS) and in Earth gravity during vertical upflow. Instabilities in microgravity are more severe than those in Earth gravity and, at their most severe, propagate to the channel's upstream region. Instabilities are observed within the channel when the intensity, I, which is dependent on inlet pressure fluctuations and mass velocity, exceeds 1.8 × 105 W/m2. Parametric trends of the frequency and amplitude of inlet pressure fluctuations during instability are examined, which reveal instabilities are most severe at low flow rates, high inlet subcoolings, and high heat fluxes. Various stability maps proposed in the literature are evaluated against the present database, and instabilities only manifest for subcooling numbers greater than 14. A subset of the database containing the Onset of Flow Instability (OFI) point is extracted to first evaluate correlations available in the literature and then develop a new correlation. The new correlation is applicable in both microgravity and Earth gravity conditions and predicts the database with a Mean Absolute Error (MAE) of 1.3%.

Numerical Investigations on Ledinegg Instability in Single and Parallel Channels under Localized Heat Source

Two-phase flow is widely used in the cooling system of electronic equipment, where the heat source is localized. However, two-phase flow may lead to flow instabilities, which have adverse effects on the system such as heat transfer deterioration and vibration. In this study, the effects of related variables on Ledinegg instability in single and parallel channels under localized heat source are investigated. Both models of single and parallel channels have been verified, and the maximum relative error is within ±5%. Based on these two models, the results indicate that the high heating power and low inlet subcooling promote Ledinegg instability, whereas Ledinegg instability is independent of the channel inclination. The average negative slope of continuous heat source, −0.09557, is smaller than that of the heat source close to inlet, −0.07408, which shows the improvement of localized heat source on Ledinegg instability. For the parallel system, when the flow excursion occurs, the total mass flow rate, which is 0.028 kg/s, is equal to two times of the mass flow rate of maximum value in branch pressure curve at which pressure is equal to 0.778 MPa, if two branches are identical. The current work provides qualitative conclusions on how Ledinegg instability will be affected with the change of relevant variables.

Study on two-phase flow instability of parallel helical tubes in steam generator of small modular reactors

The flow instability phenomenon within parallel helical tubes is experimentally studied. The oscillation characteristics of wall temperature are comprehensively discussed. In the experiment, the pressure of the test section is in the range of 2.7–6.0 MPa, the mass flux is in the range of 100–300 kg/m2s, the degree of inlet subcooling is in the range of 10–75 °C, and the inlet throttling is in the range of 82–328. This study reveals different effects of inlet subcooling. Specifically, increasing the inlet subcooling under high mass flux enhances the stability of parallel helical tubes. However, increasing the inlet subcooling under low mass flux and low subcooling weakens the stability. The oscillation period to mixture transit time ratio is small at high inlet subcooling and increases with decreasing subcooling. Furthermore, this study shows that the onset of wall temperature oscillation is closely related to the dryout phenomenon, and that the oscillation amplitude increases with increasing local quality. Finally, this study demonstrates that the flow instability can be accurately predicted using time-domain method. The simulation results show satisfactory accuracy, with the prediction errors of ± 15% for the instability threshold and oscillation period, and ± 20% for the wall temperature oscillation amplitude.

Bubble behaviors and flow boiling characteristics in open microchannels with large aspect ratio

Large aspect ratio of microchannels enables the corresponding heat sink possess higher heat transfer capability, however, the generated bubbles tend to rapidly extend forward and backward due to the large aspect ratio of the channels, thereafter blocking the channels, deteriorating the heat dissipation and increasing the pressure loss. Open microchannels have been reported to inhibit the two-phase flow instability due to the existence of extra gap for vapor expansion. However, the visualization of bubble dynamics and characterization of flow boiling behaviors of open microchannels with large aspect ratio are limited. In this study, the open microchannel heat sink with large aspect ratio was proposed and flow boiling tests were conducted at two mass fluxes of 153 and 229 kg/m2·s with the effective heat flux ranging of 180.81–2165.15 kW/m2. The inlet temperatures of the test module were set in the range of 5-85 °C. Visualization results showed that the flow patterns can be classified into bubbly flow, combined bubble flow and stratified slug flow. The flow mode tended to transition to stratified flow as the inlet temperature increased, and the combined bubble flow was found to be absent at high inlet subcooling. The size of the nucleated bubbles increased with the increase of inlet temperature during bubbly flow. The length to width ratio, survival time of nucleated bubbles and the bubble dynamic behaviors were respectively discussed. The heat transfer characteristics and pressure drop were strongly dominated by the two-phase flow patterns. The optimum flow boiling heat transfer was achieved when the flow pattern transitioned to combined bubble flow and early stage of stratified slug flow, during which bubble nucleation and thin liquid film evaporation jointly contributed to the heat dissipation. The pressure drop increased with the increase of inlet subcooling because of the greater liquid viscosity, and the sudden spurt of pressure drop was observed due to the occurrence of stratified slug flow. This work provides a preliminary investigation of two-phase flow boiling performance in large aspect ratio open microchannels, which may provide important insights of bubble dynamics or interfacial mechanics in open microchannel heat sinks.

Spontaneous condensation of carbon dioxide in flue gas at supersonic state

A cleaning method of removing CO2 from flue gas by supersonic flow is investigated. A condensation flow model is developed based on the classical droplet nucleation and growth theory. The non-equilibrium phase transition in supersonic flow is predicted by adding transport equations. The effects of converging-diverging nozzle, inlet subcooling and gas composition on CO2 spontaneous condensation in flue gas are studied. The results show that the nozzle with Witoszynski curve is the first to nucleate, at x = 0.121 m. Without considering the backpressure, the shrinkage ratio and diverging angle only affect the flow behavior behind the throat. With the increase of shrinkage ratio or diverging angle, the liquid mass fraction increases from 0.062 to 0.085 and 0.056 to 0.089 respectively, but there are hazards such as increased pressure loss and reduced nozzle flow rate. The diverging length has no significant effect on the flow behavior but the increase of length can prolong the droplet growth time. The higher inlet subcooling not only makes the supersonic flow reach the critical state in advance, from x = 0.121 m to x = 0.111 m, but also significantly improves the droplet growth rate, from 0.649 m/s to 0.854 m/s. The increase of CO2 concentration greatly enhances the mass and heat transfer process.

Unsteady non-equilibrium condensation flow of 3-D wet steam stage of steam turbine with roughness using sliding mesh method

This research develops the non-equilibrium condensation model with sliding mesh technology to solve the unsteady condensing flow inside a 3D wet steam stage of steam turbine with transient rotor-stator interaction. The maximum fluctuation of time-dependent condensation parameters is predicted. The condensation loss and entropy generation considering the off-design operation and rough blades are also evaluated quantitatively. The results showed that the secondary expansion and condensation occur near the rotor trailing edge. At design operation, the time-dependent subcooling fluctuates from −9.81 K to 8.06 K at the maximum fluctuation location. The frozen rotor method over-predicts the expansion and condensation characteristics in the steam turbine stage. Moreover, the maximum relative fluctuation of time-dependent wetness is 37.14% when it changes from 0.022 to 0.048. At off-design operation, the p-T diagram is applied to compare the expansion and condensation processes. The inlet subcooling increases by 40 K, resulting in an increase of 110.34% in outlet wetness. The phase of condensation loss with high off-design inlet subcooling is ahead of that with low off-design inlet subcooling. The fluctuation of time-dependent condensation loss with off-design inlet subcooling is about 102.28 kW. In addition, the back pressure ratio changes from 0.55 to 0.10, resulting in an increase of 190.91% in outlet wetness. The fluctuation of time-dependent condensation loss with off-design back pressure ratio can reach 112.3 kW. Besides, the maximum time-averaged entropy generation and exergy destruction due to the increase of surface roughness can reach 9.37 kJ kg−1 K−1 and 5.71 kW.

Critical Heat Flux of Confined Round Single Jet and Jet Array Impingement Boiling

This study involves experimental investigation of key parameters influencing CHF for confined round single jets and jet arrays impinging normally onto square heated surfaces. The experiments are performed using R-134a, a fluid widely used for thermal management of electronic and power devices, especially in aerospace applications. A comprehensive R-134a CHF database is acquired that considers the effects of various geometrical parameters and operating conditions. Close examination of the data trends reveals several strategies to augment CHF, such as increasing jet velocity and/or total mass flow rate and employing larger jet diameters for a fixed velocity or smaller diameters for a fixed flow rate. Higher CHF is also achieved by increasing saturation pressure for a fixed inlet fluid temperature (i.e., higher saturation pressures combined with higher inlet subcooling). Fluid exit qualities point to two different CHF mechanisms: subcooled CHF at high flow rates and saturated CHF at low flow rates. Underlying mechanisms are also propounded for two types of CHF transients: a sudden sharp temperature escalation at lower flow rates and a mild gradual increase at higher flow rates. Close inspection of the heating surface following CHF tests shows localized burnout patterns which provide significant insight into both the flow characteristics within the confinement region and the spatial distribution of surface temperature resulting from jet interactions. Statistical inference techniques are used in conjunction with the new understanding of fluid flow and heat transfer physics to formulate a new correlation form for CHF. The resulting correlation, which is based on a consolidated database of the present R-134a and previous FC-72 data, shows good prediction accuracy, evidenced by a mean absolute error of 16.66% for both fluids and over broad ranges of geometrical parameters and operating conditions.

Numerical simulation of bubble dynamics in subcooled flow boiling in a channel

In this article, we report bubble dynamics in a subcooled flow boiling in a vertical boiling channel based on numerical simulation using four-equation drift flux model (DFM4). The focus of this work is to validate the bubble phenomena computed by DFM4 in high inlet subcooling, low heat fluxes and low flow rates. The computed bubble parameters such as bubble growth time tg, bubble waiting time tw, bubble departure diameter Dd based on the available correlations were compared with the experimental results obtained from our boiling loop. We also propose an analytical solution for tg and Dd from the asymptotic growth kinetics and the force balance at the time of bubble departure. We have explained some important insights of the axial variation of bubble phenomena under varying heat flux. Unal correlation for bubble departure works extremely well to a highly subcooled boiling in low heat fluxes and low flows.

Enhanced flow boiling performance in high-aspect-ratio groove-wall microchannels

The dry-out phenomenon easily occurs on the side walls in high aspect ratio (channel depth to width ratio, AR) microchannel due to the decreasing of liquid film thickness. It is desirable to design better microchannels to address this issue. In this study, we proposed a new high-aspect-ratio groove-wall microchannel (Height=200 μm, Width=80 μm, Length=10 mm, AR=2.5), which features a series of rectangular grooves etched on the plain side walls. Flow boiling experiments were conducted in the novel groove-wall microchannel and conventional plain-wall microchannel at two inlet subcoolings of 40 and 70 °C, three mass fluxes (G) of 446, 630 and 815 kg/m2·s and wall heat fluxes of 3.8–139.5 W/cm2, using deionized water as the working fluid. The outlet of the channel is connected to the atmosphere, maintaining a saturation temperature of 100 °C. Flow visualization was captured by a high-speed camera to better understand the flow and heat transfer characteristics. Compared with plain-wall microchannel, the groove-wall microchannel demonstrates earlier boiling incipience, higher heat transfer coefficient and lower pressure drop in the two-phase flow region. It was found that the groove-wall microchannel obtains better heat transfer coefficient (HTC) at higher inlet subcooling. In comparison with plain-wall microchannel, the HTC enhancements of 25.4%, 73.0% and 55.0% were obtained respectively for the mass fluxes of 446, 630 and 815 kg/m2·s at ΔTsub=70 °C, while the corresponding HTC enhancements are 25.0%, 36.5% and 56.7% at ΔTsub=40 °C, respectively. Since the groove walls characterize the thin film evaporation especially at high heat flux, an enhanced critical heat flux (CHF) was obtained. At G = 446, 630 and 815 kg/m2·s, the groove-wall microchannel enhances the CHF respectively by 26.5%, 31.1% and 45.0% at ΔTsub= 40 °C, and the corresponding enhancements are 5.3%, 10.3% and 10.0% at ΔTsub=70 °C, respectively. Furthermore, flow reversal and boiling instability (pressure drop and wall temperature fluctuations) were drastically suppressed. Boiling instabilities are more severe at high inlet subcooling. The maximum pressure drop reductions of 27.4%, 42.0% were achieved in groove-wall microchannels for the mass fluxes of 630 and 815 kg/m2·s respectively at ΔTsub= 40 °C, and the corresponding maximum reductions are 9.07%, 23.6% at ΔTsub=70 °C, respectively. Almost no difference of the pressure drop was found in two microchannels at G = 446 kg/m2·s.

Non-equilibrium condensation of water vapour in supersonic flows with shock waves

The fluid flow and heat and mass transfer in a supersonic separator are not understood well due to the complicated interaction of the supersonic flow, swirling flow, phase transition and shock waves. In the present study, we develop a wet steam model to investigate the flow structure inside a supersonic separator with the co-existence of non-equilibrium condensation and shock waves. A study of the effect of the inlet subcooling and inlet saturation on the condensation behaviour is conducted to evaluate the performance of the supersonic separation with a focus on the shock wave. The numerical result shows that the degree of supersaturation of the water vapour can reach a maximum value of 4.28 within the designed supersonic separator and generate a peak nucleation rate of approximately 1021 kg m−3 s−1. The occurrence of the shock wave changes the equilibrium thermodynamic state, which leads to the re-evaporation of the condensed droplet. Higher inlet subcooling and inlet saturation not only shift downstream the position of the shock wave, but also induce an earlier condensation and higher liquid fraction. For the present nozzle, when the inlet subcooling and inlet saturation are about 34 K and 0.28 respectively, the shock wave intersects the region of the intense nucleation process, the non-equilibrium condensation process is terminated due to the increase of the pressure and temperature downstream the shock wave. Stronger swirling flow results in non-uniform distribution of the static pressure and decreases the nucleation rate of water vapour. The high swirling flow with a maximum swirl velocity of 150 m/s weakens the liquid fraction by 25% compared to the no swirling flow. This indicates that it is important to balance the swirling flow and condensation process to achieve an efficient performance of the supersonic separator.

Numerical investigation of the effect of inlet subcooling on flow instabilities in a parallel channel natural circulation boiling system

Some of the modern reactor concepts are based on natural circulation boiling systems. These systems are prone to flow instabilities and parallel channel systems can exhibit peculiar types of flow oscillations. In the present work, the existence and the nature of instabilities in a parallel channel natural circulation test facility (PCNCTF) has been investigated through numerical simulations using the system code RELAP5/MOD3.2. The numerical model is validated using experimental data of PCNCTF. The effect of inlet subcooling on flow oscillations in parallel channels is studied for different system pressure and input powers. Time variations of mass flow rate and void fraction in the channels are examined and the nature of oscillations is analyzed using phase portraits and Fourier power spectra. It is found that there exists a particular range of inlet subcoolings (at a fixed system pressure and input power), above or below which instabilities occur. Mixed mode of oscillations at low degrees of inlet subcoolings and superimposed density wave oscillations are observed at high inlet subcoolings.

Thermal hydraulic performance analysis of a post-CHF heat transfer test facility

Post critical heat flux (post-CHF) heat transfer may occur in loss of coolant accidents (LOCAs) in water-cooled nuclear reactors when makeup water quenches the uncovered fuel rods in the reactor core. The post-CHF regimes are characterized by low heat transfer coefficient and high wall temperature due to the vapor-wall contact. A good understanding of the hydrodynamics and heat transfer process in the post-CHF regimes, especially low-quality film boiling, such as inverted annular film boiling (IAFB) and inverted slug film boiling (ISFB), is imperative for accurately modeling the post-CHF phenomena and predicting the fuel rod cladding temperature and void fraction during accidents. This paper first discusses limitations of existing post-CHF hydrodynamics and heat transfer models/correlations and then identifies needs of post-CHF experimental data under high-flow and high-pressure conditions for model improvement. To narrow down the gap in the available experimental data, a Post-CHF Heat Transfer (PCHT) test facility is designed to perform quasi-steady-state low-quality film boiling experiments under high-pressure (ranging from 0.14 to 3.45 MPa), high-flow (ranging from 150 to 2000 kg/m2-s), and high inlet subcooling (up to 50 °C) conditions. In addition to the experimental study, a numerical model using COMSOL Multiphysics tool is developed to inform the test section design and to investigate wall heat flux and wall temperature distributions in the post-CHF regimes. The COMSOL Multiphysics simulation results show that quasi-steady-state post-CHF phenomena can be achieved in the PCHT test section. Considering thermal non-equilibrium conditions in the post-CHF regimes, a one-dimensional two-fluid model is implemented into a computer code to predict the void fraction using the wall temperature information obtained from the COMSOL simulation results. A correlation for the liquid-side Nusselt number is obtained for the IAFB regime based on previous experimental data and is implemented into the code, which results in an improvement in the agreement between the calculated void fraction and corresponding experimental data. With the new Nusselt number correlation, a parametric study of the effects of the heat flux, pressure, mass flux, and inlet subcooling on the liquid-side Nusselt number is studied. In addition, flow regime transition in the post-CHF regimes is studied by comparing the existing correlation and experimental data. Furthermore, the Weber number and liquid-side Nusselt number are calculated to examine whether there exists any correlation between these two non-dimensional numbers and the flow regime transition. The simulation results provide insights for design improvement of the PCHT test section and assist the development of a test plan for future IAFB and ISFB tests under high-pressure, high-flow, and high-subcooling conditions in the PCHT test facility.

An experimental investigation of flow boiling instability in a natural circulation loop

An experimental investigation of flow boiling instability in a single macro-channel natural circulation loop at high inlet subcooling and low pressure is performed. Flow boiling instabilities without and with flow reversal which are classified as pressure drop oscillations are observed. The characteristics of flow reversal in a single macro-channel are illustrated. Flow reversal is verified on the basis of three evidences: negative inlet flowrate, inlet water temperature pulse and large amplitude pressure oscillations in the compressible volume. The mechanism of flow reversal is given by correlating experimental heat fluxes with CHFs predicted by Umekawa correlation and Zhao correlation. It can be concluded that flow instability induces the premature of CHF. Flow reversal is caused by the onset of CHF and accompanied by periodic dryout and rewetting. Flow regime in the heated channel is depicted based on the exit water temperature oscillations, heating surface temperature profile and periodic dryout. A comparison has been made for the mechanism of flow reversal between macro-channel and micro/mini-channel. Three prerequisites contribute to the occurrence of flow reversal in a macro-channel: (1) low pressure natural circulation, (2) long two-phase region and (3) upstream compressible volume. Lastly, a stability map which distinguishes the boundaries of flow instabilities without and with flow reversal is given.

An investigation on flashing instability induced water hammer in an open natural circulation system

The flashing instability induced water hammer phenomenon (FIIWH) is firstly found in an open natural circulation experimental facility, which is used to study the thermalhydraulics characteristic of natural circulation style PCS. The primary mechanism of the FIIWH is that, the periodic movement of flashing front in the upwards channel leads to the periodic violent changes of the local pressure and temperature at the underpart of upwards channel. However, the pressure change rate is more rapid than temperature, which causes the alternately transforms of water from superheated and subcooled at the underpart of upwards channel, and results in the periodic violent evaporation and steam condensation process there. Among them, the rapid condensation causes the sudden acceleration and collision of water columns and forms a transient high pressure peak i.e. the FIIWH. Besides, the horizontal tube at the underpart of upwards channel (pipeline structure) also makes an essential effect on the FIIWH.Furthermore, the influence factors on the region and intensity of FIIWH, e.g. the heating tube outlet temperature, inlet subcooling, loop structure and non-condensable gas in water are experimental studied in detail. The initial and terminal conditions of the FIIWH are also found out by experiments. And the contour map describing the scope and intensity of the FIIWH are drawn in this paper.Finally, a simplified numerical model is carried out to simulate the pressure peak of the FIIWH. The simulation result shows a good agreement with the experimental result on the low inlet subcooling conditions, while it has a disparity with the experiment result on the high inlet subcooling conditions. And the deviation reason and improvement orientation of the model are analyzed in this paper.

Axial and radial void fraction measurements in convective boiling flows

This work is inspired by the need to have measurements and predictive capability on vertical saturated boiling flows, which are of importance in boiling water nuclear reactors. In these systems, sub-cooled water flows upwards in vertical tubes which contain a multiplicity of nuclear fuel rods, and the heat is taken away from these rods by natural convection boiling. A close analog of this situation exists when the liquid is in forced flow, and the heat flux from the vertical rods is quenched by forced convection boiling of the upward liquid water flux.In this work, we present experimental data has for axial, and radial vapor void fraction distributions in an annular boiling channel for low mass-flux forced the flow of water at high inlet sub-cooling. A single centrally placed electrical rod, designed to mimic the nuclear fuel rod has been used. The void fraction measurement is made using gamma ray densitometry technique. Axial and radial vapor void fractions have been reported, as a function of inlet liquid flux and inlet liquid temperature. The experimental data has been rationalized using a simple one-dimensional drift flux model adapted to the conditions of the experiment.

The characteristics of premature and stable critical heat flux for downward flow boiling at low pressure in a narrow rectangular channel

Flow instability and critical heat flux (CHF) especially for downward flow of water in a vertical narrow rectangular channel heated from both sides were experimentally investigated. The gap, width, heated width, and heated length of the channel were 2.35, 40, 30, and 350mm, respectively. The flow boiling was developed as the wall heat flux was increased for the imposed mass flux of 500 and 1000kgm−2s−1 for high inlet subcooling (52–74K) conditions under atmospheric pressure. In this paper, the unstable boiling flow was investigated by monitoring the channel pressure drop and high speed visualization for side view of boiling (HSV-SVB) with the back-lighting method. The results showed that an irremovable pressure drop fluctuation was involved in the flow boiling after the distinct initiating point called as the onset of pressure drop fluctuation (OPDF). We determined that coalescence of bubble (or vapor) layers on opposing heated surfaces triggered the OPDF, since an abrupt increase of pressure drop was occurred as the expansion of the liquid–vapor interface became bounded by the other layer on the opposite side. The fluctuation was amplified as the exit quality was increased and the flow regime passed through the unstable slug or churn flow. For a low inlet throttling condition, premature CHF was induced at the maximum fluctuation during those flow regimes. On the other hand, stable CHF was obtained in the re-stabilized flow regime as the annular flow was developed only for large inlet throttling conditions with an additional pressure drop across the valve of more than 0.4bar for our experimental configuration.

Investigation of two-phase flow instabilities under advanced PWR conditions

The characteristics of two-phase flow instability under advanced pressurized water reactor (PWR) conditions are explored in this study. Density wave oscillations in vertical parallel tubes are experimentally investigated by employing three different test sections to observe the parametric effects of the axial power shape, unheated riser, and channel inlet throttling on the onset of flow instability (OFI). A critical heat flux (CHF) accompanied by flow oscillation is indicated under certain test conditions with high pressure and/or low inlet subcoolings. A frequency-domain linear stability analysis code, called ALFS_F, was developed on the basis of one-dimensional two-phase mixture equations and a reactivity feedback model with point neutron kinetics and lumped parameter heat conduction equations. The mass flux at OFI conditions and the oscillation periods are calculated by the ALFS_F to compare with the experimental data. The thermal margin characteristics are illustrated with a comparative analysis of OFI and CHF margins under the core conditions of SMART, which is an integral-type advanced PWR being developed at KAERI.

Critical heat flux of cyclohexane in uniformly heated minichannels with high inlet subcooling

The critical heat flux (CHF) for flow boiling of cyclohexane was experimentally investigated in electrically heated minichannels with inside diameters of (1 and 2)mm, and heated lengths of (360 and 710)mm at mass velocities ranging from (318 to 1274)kg/m2s, heat fluxes from (50 to 1000)kW/m2, and pressures of (1.0, 2.0, 3.0)MPa. Cyclohexane entered the test channel at ambient temperature of about 20°C, equivalent to high subcooling of (162.1, 206.4 and 236.3)°C at (1.0, 2.0 and 3.0)MPa respectively. The critical vapor qualities ranged from −1.35 to 0.83. Results indicated that CHF of cyclohexane increased with increasing mass velocity and decreasing heated length. Reducing channel diameter at the same length-to-diameter ratio was proved to have no effects on CHF values of cyclohexane at the evaluated conditions. CHF mildly increased with an increase in pressure at higher mass velocities, but critical vapor quality always decreased with the increasing pressure. Experimental CHF data were compared with the Katto–Ohno (1984) correlation and Shah (1987) correlation. Good agreements were achieved between the experimental data and those predicted with most deviations not exceeding 30%. A corrected Katto–Ohno CHF correlation is developed to fit the current experimental data. Excellent agreement was obtained with all the deviations (whatever of saturation CHF or subcooled CHF) in the range of ±10% with RMS error of 5.35%.

Control of density wave oscillations in boiling channel

The present study reports uncontrolled behavior and controlled behavior of boiling flow in a mini-channel excited by step perturbation and periodic perturbation. It also demonstrates the performance of nonlinear model predictive control (NMPC) in reducing the chaotic oscillations. The control performance significantly varies when the control region is changed from the supercritical bifurcation (stable periodic orbit) region at high inlet subcooling to the subcritical bifurcation (unstable periodic orbit) region at low inlet subcooling. The study also includes the effect of forcing amplitude and forcing frequency on the uncontrolled and controlled dynamics of the channel.

Theoretical investigations on two-phase flow instability in parallel channels under axial non-uniform heating

Two-phase flow instability in parallel channels heated by axial non-uniform heat flux has been theoretically studied in this paper. The system control equations of parallel channels were established based on the homogeneous flow model in two-phase region. Semi-implicit finite-difference scheme and staggered mesh method were used to discretize the equations, and the difference equations were solved by chasing method. Cosine, bottom-peaked and top-peaked heat fluxes were used to study the influence of non-uniform heating on two-phase flow instability of the parallel channels system. The marginal stability boundaries (MSB) of parallel channels and three-dimensional instability spaces (or instability reefs) under different heat flux conditions have been obtained. Compared with axial uniform heating, axial non-uniform heating will affect the system stability. Cosine and bottom-peaked heat fluxes can destabilize the system stability in high inlet subcooling region, while the opposite effect can be found in low inlet subcooling region. However, top-peaked heat flux can enhance the system stability in the whole region. In addition, for cosine heat flux, increasing the system pressure or inlet resistance coefficient can strengthen the system stability, and increasing the heating power will destabilize the system stability. The influence of inlet subcooling number on the system stability is multi-valued under cosine heat flux.