Two-phase Flow Instability Research Articles

Experimental investigation of flow boiling instability and associated pressure fluctuations in a mini-channel heat sink

Two-phase flow instability, inherently accompanied by hydraulic and thermal oscillations, is a complex phenomenon that must be overcome to design an efficient flow boiling heat transfer system. The potential factors influencing two-phase flow instability are so diverse and complex that many efforts have been devoted to identifying and classifying flow instabilities. Density wave oscillation and pressure drop oscillation in a single channel represent classical flow instabilities, and their mechanisms are relatively well established. However, parallel channel instability observed in multi-channel heat sinks is challenging to analyze due to flow interaction between neighboring channels through the inlet/outlet plenum. Even the number of channels, the size and shape of the plenum, and the inlet/outlet arrangement influence the fluid flow in multi-channel heat sinks, complicating the analysis of previous experimental data from the literature. Therefore, to address the complexity of multi-channel flow boiling and enhance the understanding of its behavior under specific conditions, additional experimental studies are essential. This study conducts flow boiling experiments using FC-72 with a mini-channel heat sink consisting of 22 parallel channels, each with dimensions of 1.6 mm in width and 4.8 mm in height, operating under two distinct pressure conditions. Flow instability occurring within the present channel flow boiling is investigated by visualizing the flow in the channel upstream and inlet plenum, as well as through spectral analysis of the pressure fluctuation. The fast Fourier transform was used to characterize the oscillation characteristics of transient pressure measured at various locations in the test loop. In addition, visualization data and fast Fourier transform results identified flow oscillation caused by vapor backflow in the channel. As a result, the present flow boiling data are classified as stable or unstable based on the frequency of pressure oscillation, and a new stability map for mini-channel flow boiling is proposed.

Experimental investigation of pressure drop oscillation and active mitigation in a pumped two-phase loop

A pumped two-phase loop (P2PL) integrated with a microchannel evaporator is the most favored design for advanced thermal management of modern electronic devices. However, these systems are susceptible to two-phase flow instabilities, in particular pressure drop oscillation (PDO), which can compromise their performance by premature initiation of critical heat flux (CHF), deterioration of the heat transfer capabilities, and inducing severe thermal and pressure fluctuations. This study experimentally investigates PDO in a P2PL with a microchannel evaporator, exploring the underlying mechanisms and mitigation strategies. The range of parameters are: heat flux from 0 to 56 W/cm2, mass flux from 47.9 to 95.9 kg/m2-s, a fixed inlet temperature of 10 °C, initial accumulator gas pressure from 620.5 to 827.3 kPa, and vapor quality from zero (subcooled liquid) to one (pure vapor close to dryout). A consistent trend of high-amplitude, low-frequency PDO at low heat fluxes transitioning to low-amplitude, high-frequency PDO at high heat fluxes was observed across all operating conditions. PDO originating in the evaporator propagated to other loop components with similar characteristics, underscoring the systemic nature of PDO. Further, PDO-driven thermal oscillations were observed to propagate through the evaporator, reaching the heater (heat source) temperature with maximum amplitude of 3.4 °C. These oscillations had a detrimental impact on the heat transfer coefficient by altering the flow boiling regimes within the microchannels. Notably, the subcooled liquid temperatures remained stable, indicating the absence of pressure-induced thermal effects in single-phase regions. Finally, an active control technique, based on controlling the pump flow rate using continuous feedback of flow rate was implemented to mitigate PDO, eliminating both pressure and temperature oscillations, achieving an average reduction of 69.8 % in PDO amplitude.

Experimental Study of Single and Two-Phase Flow and Heat Transfer Characteristics in Helical Tube under Motion Condition

The unique structural characteristic of the helical coiled pipe causes some special phenomena such as the secondary flow effect inside the tube, and the heat transfer characteristics are more complicated than those in circular pipe. There have been few published papers about experimental research of flow and heat transfer characteristics of helical tube under motion conditions. An experimental loop was built based on the background above. An experimental study of single-phase and saturated boiling two-phase heat transfer characteristics and flow instability in helical tubes under multiple motion conditions was carried out. In which the maximum system pressure was 4 MPa, mass flux was 550 kg/(m2·s), and heat flux of the test section was 360 kW/m2. The results show that the heat transfer capacity of the side close to the central axis of the helical tube is significantly lower than that of the side away from the central axis under rocking and inclination conditions. An empirical correlation of the heat transfer coefficient in helical tube under static and motion conditions was drafted based on the experiment data of single-phase conditions. The influence of motion conditions on two-phase heat transfer characteristics was similar to that of a single phase but much weaker.

Numerical study on flow instability in parallel helical tubes under ocean conditions

The superimposed coupling of flow oscillations generated by transient external forces under ocean conditions and flow oscillations in parallel helical tubes during two-phase flow instability may lead to more complex flow oscillation phenomena in parallel helical tubes. However, there is still a lack of understanding regarding the effect of ocean conditions on the two-phase flow instability within parallel helical tubes. The two-phase flow instability in parallel helical tubes under ocean conditions is numerically investigated in the present study. The effects of ocean conditions on the flow behavior and instability threshold in parallel helical tubes are studied with mass flux 200 kg/m2s and inlet subcooling within 5–70 °C. The results show that the mass flow rate decreases as the inclination angle increases. A larger rolling amplitude or shorter period increases the flow and temperature oscillation amplitude. Two characteristic frequencies are found in the natural circulation flow instability under the rolling motion. As the rolling amplitude increases, the compound flow oscillation amplitude is larger and more chaotic with a certain phase shift. The effect of stationary inclining on the flow instability threshold is rather significant than rolling effect. The results of the present study can serve as a significant reference for the operation and design of the helical-coiled once-through steam generator under ocean conditions.

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%.

Flow boiling heat transfer enhancement of R134a in additively manufactured minichannels with microengineered surfaces

Additive manufacturing (AM) of metal components has opened new frontiers in heat transfer applications owing to its wide design freedom, which enables previously unexplored geometries with enhanced thermal performance to be developed. Open minichannels, on the other hand, consist of an extra manifold situated above the common flow channels, have been demonstrated to effectively reduce pressure drop and two-phase flow instability by mitigating backflow and maldistribution. In this paper, we synergized open minichannel design methodology with an ultrascalable surface microstructuring method for AM AlSi10Mg to improve flow boiling heat transfer performance of R134a. Experiments were conducted at the refrigerant mass flow rates (ṁ) of 0.005 kg/s to 0.01 kg/s (corresponding to mass fluxes of 73 to 146 kg/m2⋅s), and effective heat fluxes (qeff) of 1.8 kW/m2 to 141 kW/m2, by supplying 2 ℃ subcooled liquid to the minichannel inlet at saturation pressure (Psat) of 7.27 bar. The effects of heat flux, nucleation site density, mass flow rate and location of nucleation sites on the harmonic-average heat transfer coefficient (have) and pressure drop (ΔP) along the flow direction are investigated and compared against a conventionally manufactured (Al6061) counterpart. Our results show that the proposed etching process significantly improves the cooling performance of AM microstrucured minichannels, resulting in 210% higher have than Al6061 with negligible pressure drop penalty. The flow visualization results reveal a sequential order of nucleation site activation with increasing heat flux for microstructured open minichannels, which starts from the top of fins, followed by the corners and other three surfaces within the channels. This also results in the non-negligible effects of mass flow rate on cooling performance for microstructured minichannels, with approximately 10%–66% higher have at the lowest mass flow rate. Besides, the pressure drop penalty resulting from plain AM rough surface is also reduced by up to 13% through the proposed microstructuring process. In all, this work not only successfully identifies the dominant mechanisms in AM microstructured open minichannels, but it also provides useful minichannel design guidelines for high-performance two-phase cooling devices by AM.

Numerical study on two-phase flow instability in multi-parallel channels of helical-coiled once-through steam generator of lead-cooled fast reactor

The helical-coiled once-through steam generator (H-OTSG) has the advantages of compact structure and strong heat transfer ability, and is appropriate for lead-cooled fast reactor. The two-phase flow instability may cause mechanical vibration and thermal fatigue of heat transfer tube bundles, posing a serious threat to the safe operation of the H-OTSG. In this work, the secondary side of the H-OTSG characterized as multi-parallel channels is modeled by RELAP5/MOD3.4 code, and the oscillation behavior during start-up and the influence of structural and operating parameters on system stability are studied based on time-domain method. The results indicate that the pressure, flow rate, and temperature of the secondary fluid exhibit density wave oscillations at the heating section in a (n-2,2) pattern. In addition, increasing inlet throttling, reducing outlet throttling, controlling subcooling within an appropriate range and avoiding operation under low loads are all beneficial for improving the system stability of the helical-coiled once-through steam generator.

Investigation on spray and flame stabilization of a LOX/methane swirl coaxial injector

Variable-thrust cryogenic propellant rocket engines are gaining significant research interest in space exploration. However, low-frequency unstable combustion is still a challenging scenario, especially in fuel-rich preburner and low-thrust operating conditions of deep variable thrust. To deeply understand the mechanism of low-frequency unstable combustion, chemiluminescence images of CH* and background light images of the spray were obtained synchronously by chemiluminescence imaging and laser background light imaging, respectively. Both the spray and flame stabilization of liquid oxygen/methane swirl coaxial injector were studied through the continuous regulation of liquid oxygen mass flow rate. The results showed that low-frequency unstable combustion occur in both the start-up stage and the throttled stage under the fuel-rich condition at a frequency of 39.1∼48.1 Hz and an amplitude of 30% of the average combustor pressure. It is found that the two-phase flow instability of liquid oxygen is likely to induce spray and flame instability, resulting in low-frequency unstable combustion. The dimension subcooling degree of liquid oxygen is an important factor affecting unstable combustion. When the dimensionless subcooling degree is larger than 0.7, the low-frequency unstable combustion is suppressed. On the other hand, as the mixing ratio decreases, the flame oscillation mode gradually transforms from the longitudinal oscillation mode to contraction/expansion mode. Flame filling up and flashback processes can be observed in both flame oscillation modes, and the entropy coupling mechanism of the oscillation mode is explained in detail. Furthermore, an oscillation period is determined to include four processes: propellants filling up and flame liftoff; heat release of the combustion products and entropy disturbance; acceleration of the entropy wave through the nozzle creating an acoustic disturbance; and flame flashback, in which the heat release time of combustion products and entropy disturbance is the longest.

Influence of heat flux profile on two-phase flow instability in parallel channels of helical coil once-through steam generator

To enhance the heat transfer capability, the helical coil once-through steam generator (HCOTSG) has been widely utilized in the small modular reactor (SMR). There are hundreds of parallel helical channels in the HCOTSG, and the coolant undergoes the phase change from subcooled liquid to superheated vapor in the helical tube. The flow oscillation between parallel channels may be triggered during the phase change. In present study, the two-phase flow instability is investigated with different heat flux profiles under many operating conditions. The thermal–hydraulic model for parallel channels of HCOTSG is established based on our previous work, while the local heat transfer phenomenon such as subcooled boiling, critical heat flux, and wall conduction are considered in detail. The heat flux profile along the tube is ascertained by the steady-state analysis of HCOTSG under different operating conditions and applied to the outside surface of the helical channel. The heat flux value is increased step by step to trigger the out-of-phase oscillation between parallel helical channels. The thermal–hydraulic model and analysis method are validated against experimental results. The characteristics of flow instability in the helical channel are also analyzed. It is observed that the delay effect causing the flow instability is more obvious if the void fraction exceeds 0.7 in the starting stage. Then the boundary of flow instability is compared under different heat flux profiles for various operating conditions of HCOTSG. The results suggest that the distribution of heat flux in the two-phase region plays a more significant role in the occurrence of flow instability in the parallel helical channel system. The thermal–hydraulic characteristics during flow instability are governed by the combined effects of void propagation and delayed feedback in the system. In conclusion, the thermal–hydraulic condition of HCOTSG undergoes the unstable region to achieve the normal operating point, which can be avoided by adjusting the operating parameters.

System code validation in periodic two-phase flow from low-pressure oscillations

Low-pressure two-phase flow instabilities can potentially challenge start-up transients of water-cooled nuclear reactors. Predicting oscillations caused by flow instability is therefore paramount to reactor safety. While many thermal hydraulics system codes are developed as general-purpose tools, their performance is not guaranteed outside their optimized application ranges. The current study therefore performs validations of two thermal hydraulics system analysis codes, the Adaptive SYStem Thermal-hydraulics Version 3 (ASYST VER3) and Reactor Excursion and Leak Analysis Program MOD3.3 (RELAP5/MOD3.3), in reproducing oscillatory two-phase flow induced by low-pressure flashing instability. Benchmark conditions are selected from a novel dataset resolving void fraction with radial resolution and quantifying periodic behavior with ensemble averaging. Focusing on the prediction of transient two-phase phenomena rather than the determination of stability, validations are conducted by simulating a single channel under prescribed periodic boundary conditions. ASYST exhibits a systematic underprediction of void fraction in an adiabatic chimney downstream of a heated section. The prominent causal discrepancy is identified as a lost travelling void wave due to overpredicted condensation. RELAP5 noticeably overpredicts subcooled boiling and underpredicts flashing, which is consistent with existing validations against steady-state separate-effect tests under low pressure. This degraded code performance also suggests that in low-pressure transients prone to flashing instability, the confidence in RELAP5 derived from existing validations against integral-effect tests shall be carefully limited to validated capabilities in integral systems. In general, the current study fills the previous gap of knowledge about the performance of ASYST and RELAP5 in predicting detailed transient two-phase phenomena in low-pressure flashing-induced oscillations. The identified code defects reveal future directions for code improvements eventually towards reliable application of ASYST and RELAP5 under low pressure beyond their original calibration.

Experimental investigation on two-phase flow instability induced by direct contact condensation in open natural circulation

Direct contact condensation (DCC) is a phenomenon that occurs frequently in nuclear and steam power systems, causing two-phase flow instability and water hammer events. This, in turn, results in system oscillations and, in some cases, leads to structural failure. This paper aims to characterize and highlight the mechanisms and features of the DCC-induced two-phase flow instability in an open natural circulation system employing visual experiments. To aid this, a laboratory-scale experimental rig is established based on the working principle of open natural circulation. In the experiments, the transient two-phase flow images are captured by a high-speed camera. Five kinds of flow instability modes and evolution characteristics are analyzed quantitatively and qualitatively using image processing techniques. The results indicated that the frequency of instability is influenced by the amount of heating power, and the instability mode is mainly influenced by the subcooled-water temperature. Moreover, the steam condensation rate was found to have a direct impact on the instability extent. These results obtained in this study offer insights into the full characterization and understanding of two-phase flow instability in open natural circulation systems, as well as towards the prevention of DCC-induced water hammer in offshore nuclear power ships.

Influence of convective heat transfer and wall thermal capacity on dynamic interactions between wall temperature and pressure drop oscillations during microchannel flow boiling

Flow boiling in microchannel heat sinks is attractive for use in industrial thermal management applications owing to highly effective heat dissipation and compactness. However, flow boiling is subject to two-phase flow instabilities, specifically pressure drop oscillations, that may deteriorate performance by causing transient wall temperature oscillations or initiating premature critical heat flux. While there has been extensive model-based investigation of the stability and flow dynamics associated with pressure drop oscillations in microchannels, prior work has not focused on the coupled interactions between these flow oscillations and the dynamic heat transfer processes, as is required to predict thermal performance degradation and ensure reliable operation. This study predicts the influence of wall thermal capacity and heat transfer coefficient on the dynamic interaction between pressure drop oscillations and wall temperature. First, we demonstrate and explain how added wall thermal capacity acts to reduce the amplitude of pressure drop oscillations due to coupling between the thermal and flow dynamics. Whereas previous work has attributed this effect to a thermal time constant of the wall, the amplitude of the pressure drop oscillations is shown to scale with the Biot number by scanning various combinations of wall thermal capacity and heat transfer coefficient. A Biot number threshold is thereby suggested as the criterion for judging whether the thermal wall capacity must be considered in dynamic models, based on the strength coupling between the wall temperature and pressure drop oscillations.

Experimental Investigation on Boiling Heat Transfer Characteristics of Liquid Methane in Mini Channel

LOX/LCH4 rocket engine has been recognized as the ideal power choice for future space vehicles due to the merits of low cost, non-toxic and pollution-free, convenient maintenance, suitable for reuse and high specific impulse. In the process of wide range variable thrust of LOX/LCH4 rocket engine, the coolant methane is in a subcritical state due to the low combustor pressure under low operation conditions. The instability of two-phase flow is easy to occur in regenerative cooling channel (RCC), and it is urgent to investigate the heat transfer performance of methane with phase change in RCC. Experiments have been conducted to investigate the flow boiling characteristics of liquid methane in the single mini channels with the diameters of 1.0, 1.5 and 2.0 mm. Effects of the mass flux (266.75~1781.26 kg/m2·s), inlet pressure (0.56~4.24 MPa), heat flux (53.25~800.07 kW/m2) and channel diameter (1.0~2.0 mm) on the flow boiling heat transfer coefficients are tested. Results show that there are two regions with different heat transfer mechanism, one is the nucleate boiling dominated region for low mass quality and the other is the convection evaporation dominated region for high mass quality. A new correlation expressed by Bo, We, Kp, X, Co, Ftg is proposed, which yields good fitting for 355 experimental data with a mean absolute error (MAE) of 10.9%. Present experimental results can provide reference for the thermal protection prediction and optimal design of RCC in LOX/LCH4 rocket engine.

Flow boiling heat transfer and pressure fluctuation characteristics in a divergent channel with inclined downward-facing heater

The core catcher cooling system is used in nuclear power plant to mitigate the core meltdown accident through flow boiling heat transfer. In order to better understand the flow boiling characteristics in the system, experimental research was conducted on an inclined channel. The flow channel is oriented 15° with a divergent downward-heating surface. The two-phase flow behavior and pressure fluctuations were capture by a high-speed camera and pressure transducer. The inlet mass flux and subcooling effects on flow boiling characteristics were deeply investigated. The inlet mass flux and subcooling conditions are within 100–400 kg/m2s and 1–30 K, respectively. The results showed that the boiling curves shift to the higher wall superheat with the increase of mass flux, and the effect of subcooling is weak. The bubble dynamic images by the high-speed camera showed that the bubbles continue to grow and coalesce during the sliding process along the wall. Large bubble blankets are formed periodically upon the boiling surface beyond a heat flux threshold, and they would undergo a rapid condensation process after departing the heating area, which leads to the liquid reversal and accompanied by strong pressure fluctuations. Further, beyond a certain subcooling and heat flux, two-phase flow instability phenomenon appears, which is characterized by complete condensation of bubble blanket accompanied by pressure shocks, meanwhile, the heat transfer coefficient decreases and the wall superheat increases sharply. The bubble blanket frequency could be obtained by the Fast Fourier Transform analysis of pressure, which is positively correlated with the heat flux and is negatively correlated with the mass flux and subcooling. Furthermore, a dimensionless correlation of reduction factor (RF) was proposed, which directly related the bubble blanket characteristics to the thermal–hydraulic parameters.

Experimental and numerical study of flow boiling heat transfer characteristics in rectangular groove-wall microchannels

Flow boiling in microchannels shows high potential to cope with the high power density dissipation for electronic devices. However, it is a challenge to promote the heat transfer performance, particularly temperature oscillations and critical heat flux (CHF) due to the two-phase flow instability. Here, we have made a comprehensively comparison studies on the temperature oscillation, the bubble nucleation, the flow pattern of the groove-wall microchannels and plain-wall microchannels via experimental and numerical methods simultaneously. The experimental results show that the groove-wall microchannel achieves better heat transfer performance. which were conducted at inlet subcooling of 25 °C, effective heat fluxes of 1.82–70.83 W·m−2, and mass fluxes of 819.4–3493.1 kg·m−2·s−1. The groove-wall structure can enhance the CHF by 31.3 % at mass flow rate of 819.4 kg·m−2·s−1 comparing that of plain-wall structure. The whole process of the growth movement of bubbles in the grooved area were obtained by a three-dimensional, transient numerical model using Volume of Fluid (VOF) method, which shows that the groove-wall structure can advance the incipience of boiling, and the grooved structure can help to keep the wall wet in the slug flow and annular flow area. In addition, compared with plain-wall structure, the temperature oscillation amplitude in the groove-wall structure is reduced by 77.2 % at mass flow rate of 819.4 kg·m−2·s−1, which means the groove-wall structure plays good performance in preventing the counterflow and reduces the flow oscillation in the parallel channel. This work can deepen the understanding of flow boiling mechanism in rectangular groove-wall microchannel.

Natural circulation oscillation in a seawater loop during startup

Aiming to improve nuclear safety by using seawater as an alternative emergency coolant during an accident, the knowledge of two-phase flow instabilities and heat transfer performances of seawater is needed. Startup experiments from the quiescent state at room temperature of 20–23 °C are carried out by raising heating power with 3.5 wt% artificial seawater in a natural circulation loop. De-ionized water is adopted as well for the comparative study. In this paper, a type of natural circulation oscillation at the modified heat fluxes of 383.70–530.62 kW/m2 and 404.05–594.97 kW/m2, respectively, for seawater and de-ionized water, with intermittent inlet mass flow rates is exclusively investigated, as a part of the experimental studies of two-phase flow instabilities during the startup of a natural circulation loop. Visualizations of two-phase flow behaviors at the power of 2206 W in seawater reveal the unique bubble foam regimes at the outlet of the heated section, tiny bubbles at the inlet of the riser, and the possible elongated slug bubbles at the central region of the riser.The natural circulation oscillations for two fluids are characterized by four distinct stages in sequence: calm state (stage 1), accumulations of foam/slug bubbles in seawater/de-ionized water (stage 2), propagations of foam/slug bubbles in seawater/de-ionized water (stage 3), and restorations (stage 4). The same period for stages 1, 3, and 4 for both fluids are 25 s, 4 s, and 5 s, respectively. For stage 2, the period measured as 12.5 s in seawater is significantly shorter than that recorded as 45 s in de-ionized water, leading to earlier systematic natural circulation rates. It is possibly caused by the much stronger accumulation effect of tiny bubbles in the riser, resulting in vast buoyant forces by draining more liquid out of the riser. The intermittent peak inlet mass flow rates of 0.017 kg/s and 0.023 kg/s are recorded, respectively, for seawater and de-ionized water. It would not induce a pressure drop increase over the riser while cause a very high pressure drop oscillation of 4.67 kPa in de-ionized water. Slower circulation rates induce smaller wall temperature drops of 3.6 °C in seawater than that of 5.2 °C in de-ionized water. Under zero flow rate conditions, the wall temperatures are mainly kept at 117–119 °C, accompanied by bubble foam without ruptures in seawater, while that are about 115 °C with slug bubbles due to bubble coalescences in de-ionized water. It demonstrates that the immobile foam may deteriorate the heat transfer under pool-like conditions.

Two-phase flow instability characteristics of HTGR Once Through Steam Generators

High Temperature Gas-cooled Reactors (HTGR) have the advantage of inherent safety compared with most Gen III reactors. They also have very high temperature, which make the plant have high electricity generation efficiency and have has the potential for hydrogen production. Once Through Steam Generator (OTSG) is responsible for transferring high temperature heat from primary loop helium to secondary loop composed of water and steam. The evaporation tubes of HTGR OTSG are very long due to the low heat transfer coefficient of helium convection over tube bundles. A Two-phase flow instability may occur in HTGR OTSG, which can cause pulsation of the mass flow rate, system pressure and temperature. This will not only interfere with the control system, but also cause thermal fatigue damage in the structures. Thus, it is important to predict and avoid two-phase flow instability in HTGR OTSG. Three types of flow instabilities are investigated for HTGR OTSG using both a linear analytical model and a nonlinear numerical model. They are flow excursion (or Ledinegg instability), density wave oscillation and pressure drop oscillation. For the nonlinear numerical model, RELAP5 software is adopted. Flow excursion and pressure drop oscillation are discussed based on the predicted hydraulic characteristic curve of HTGR OTSG (pressure drop vs. mass flow rate curve). Density wave oscillation is investigated using the transient variation of the outlet mass flow rate. For the linear analytical model, a set of ordinary differential equations are established using integration and small perturbation methods. The variables of differential equations are the length of the subcooling water region, the length of both the subcooling water and saturated two-phase regions, the average density of the saturated two-phase region and the inlet mass flux. The stability of OTSG is investigated using the eigenvalues of the coefficient matrix of the ordinary differential equations. Based on the results calculated using the linear analytical model and nonlinear numerical model, there is no flow excursion, pressure drop oscillation or density wave oscillation at the designed partial and full power levels (30%, 50%, 75% and 100%). Thus, HTGR OTSG can operate stably and have a sufficient safety margin at the designed power levels. Density wave oscillation will happen when decreasing system pressure. However, increasing inlet resistance coefficient can avoid density wave oscillation effectively. The tube lengths’ influences on the stability boundary of OTSG are also investigated and it shows no strong dependency.

Theoretical analysis about model simplifications and boundary conditions on two-phase flow instability of parallel heated tube systems

Flow instability may happen in both subcritical two-phase flow boiling systems (such as U tube natural circulation steam generator of Pressurized Water Reactor, once through steam generator of High-Temperature Gas-Cooled Reactor, etc) and supercritical evaporation systems. In previous theoretical researches, the model of a single heated tube (SHT) with constant pressure drop boundary condition (ConPD) was always used to investigate the system stability. However, in real applications, there are usually multiple parallel heated tubes (MHT) and unheated connection regions before the branch heated tubes. The system is usually driven by a pump, and there is always a long unheated main pipe and throttling between the heated tubes (boiler or steam generator) and the pump. However, there lacks rigorous analysis about the effects of these model simplifications and boundary conditions. Take superheated two-phase flow boiling systems as examples, the theoretical models with different flow-driven boundary conditions (ConPD, constant mass flow rate (ConMF), and pump driven (PumpD)), different numbers of heat tubes (SHT, MHT), with or ignoring the unheated region (UHR), with or ignoring the bypass of a pump, with identical or un-identical geometry parameters and operation parameters are established. Linear ordinary differential equations describing the instability are obtained. Eigenvalues of the coefficient matrix of linear ordinary differential equations, and the property of the block matrix and the similarity matrix are used to analyze the effects of the above boundary conditions and model simplifications on the system stability. Two new dimensionless numbers, dimensionless pump number (Npump) and dimensionless bypass number (Nbypass), are proposed to describe the effect of pump and its bypass on stability quantitatively. The effect of the pump bypass on the stability can be attributed to the decrease of Npump. No matter ConPD or PumpD is adopted, SHT considering UHR is proved to be more stable than SHT ignoring UHR. No matter UHR is considered or ignored, SHT with PumpD is proved to be more stable than SHT with ConPD. No matter UHR of MHT at the main pipe is considered or ignored, the stability of MHT with ConPD or ConMF or PumpD is proved to be the same as that of SHT with ConPD ignoring UHR. When ConPD ignoring UHR is adopted, the stability of MHT with un-identical geometry and operation parameters is proved to be the same as that of its most unstable heated tube. The above conclusions about the effect of model simplification and boundary condition are obtained from superheated two-phase flow boiling systems, they also hold for saturated two-phase flow boiling systems and supercritical evaporation systems.

Experimental research on cooling performances of radial expanding-channeled heat sinks applied for multiple heat sources

Intrinsic two-phase flow instability using multiple parallel boiling channels hinders the actual implementation of flow boiling cooling techniques in high heat flux and multiple heat source applications. Recently, we proposed a radial expanding-channeled heat sink (ECHS) to realize a high two-phase flow stability under extreme conditions. In the present work, the cooling performances of two heat sinks connected in series or in parallel were investigated with the heat load adjusted individually. The results indicated that when two ECHS were connected in parallel, the inlet flowrate was evenly distributed into the two heat sinks, and their heat transfer performance was identical. At a sudden change in the heat load of one path, the flowrate of the other heat sink may change by 10% to 30% without inducing flow instability or heat deterioration. The thermal resistance for both heat sinks was lower than 0.025 °C/W, and no instability or dry-out was observed as long as the initial flowrate was larger than 0.28 kg/min. With the proposed ECHS, multiple heat sources such as central processing unit, server, rack, etc. can be connected in series or in parallel to construct an integrated cooling system for large-scale data center applications.

A microfluidic evaporator with a photothermal porous layer for continuous sample concentration

On microfluidic platform, sample concentration plays a crucial role in on-chip detection, analysis and synthesis, etc. Evaporation-induced solvent removal is frequently applied to various application scenarios due to its simplicity and compatibility. However, the enclosed microfluidic channels often hinder the solvent vapor discharge during the evaporation process due to the formation of two-phase flow and interfacial instability. Herein, we propose a novel microfluidic evaporator incorporated with a photothermal porous layer for continuous sample concentration. By absorbing the sample fluid into the porous structure, the evaporator is capable of achieving volumetric heating of the sample fluid thus enhancing the solvent evaporation while the porous network ensured efficient vapor venting. Moreover, the thermal resistance of porous layer shields the heat transfer towards the bulk fluid therefore prevented the sample solution from overheating during the operation process. The results demonstrate the designed microfluidic evaporator exhibits good sample concentration capability under continuous flow conditions.