Degree Of Subcooling Research Articles

Morphology Study of Hydrate Shell Crystal Growth on a Microbubble Interface in the Presence of Additives

Bubble dispersions in water can be observed in both wellbore or subsea piping as well as subsea gas seepages. The hydrate risk in subsea pipelines or methane leaks from the cold spring into seawater can be strongly influenced by the hydrate crystal growth on bubbles. In addition, the bubbling method has been proven to be an effective method to enhance the gas–liquid mixing pathway and promote hydrate’s rapid formation in the application of hydrate-based technologies, yet the microscopic mechanism of hydrate film growth on the bubbles still needs to be further clarified. In this study, the hydrate crystal growth morphology evolution at the microbubble interface was investigated by a high-resolution microscope combined with the temperature-controlled stage. The results showed that the hydrate crystals nucleated rapidly at the bubble interface to form a hydrate shell-coated structure in a pure water system. The defect growth mechanism dominated the hydrate crystals growth at the bubble interface in additive solutions. Hydrate did not nucleate directly on the surface of bubbles but by trapping guest molecules around the shrinking bubbles to form aggregated hydrate agglomeration in the presence of surfactants. In addition, the bubbles tend to provide a steady stream of gas source for hydrate formation. The growth of hydrate shells contributed to a normal distribution of bubble shrinking rates, with an average rate of 9 μm/s in 0.5 wt % NaCl solution. The existence of a high concentration of ions or organic molecules at the microbubbles’ interface and the exclusive effect played a dominant role in the process of hydrate formation. In addition, it was found that the repulsive effect of ions at the bubble interface on hydrate formation was weak under a high subcooling degree. However, other organic molecule promoters still accelerated the hydrate’s formation rate because they increased the dissolution and diffusion rate of gas in solution. The results are expected to strengthen further the applications relating to hydrate-based phase transition promotion technology in the industry in energy storage, gas separation, or inhibition techniques in flow assurance of the offshore industry.

Preferred Mode of Atmospheric Water Vapor Condensation on Nanoengineered Surfaces: Dropwise or Filmwise?

Condensing atmospheric water vapor on surfaces is a sustainable approach to addressing the potable water crisis. However, despite extensive research, a key question remains: what is the optimal combination of the mode and mechanism of condensation as well as the surface wettability for the best possible water harvesting efficacy? Here, we show how various modes of condensation fare differently in a humid air environment. During condensation from humid air, it is important to note that the thermal resistance across the condensate is nondominant, and the energy transfer is controlled by vapor diffusion across the boundary layer and condensate drainage from the condenser surface. This implies that, unlike condensation from pure steam, filmwise condensation from humid air would exhibit the highest water collection efficiency on superhydrophilic surfaces. To demonstrate this, we measured the condensation rates on different sets of superhydrophilic and superhydrophobic surfaces that were cooled below the dew points using a Peltier cooler. Experiments were performed over a wide range of degrees of subcooling (10-26 °C) and humidity-ratio differences (5-45 g/kg of dry air). Depending upon the thermodynamic parameters, the condensation rate is found to be 57-333% higher on the superhydrophilic surfaces compared to the superhydrophobic ones. The findings of the study dispel ambiguity about the preferred mode of vapor condensation from humid air on wettability-engineered surfaces and lead to the design of efficient atmospheric water harvesting systems.

Study on the Dynamic Generation of Subcooled Water Using a Compact Heat Exchanger

The dynamic generation of ice slurry from subcooled water is one of the most promising ways to make ice; this process is utilized widely in ice storage air-conditioning systems. However, the random occurrence of ice blockage during the generation of subcooled water using conventional heat exchangers prevents the increase in subcooling, thereby reducing the efficiency of the release of the subcooled water and converting it into ice slurry. A more efficient approach to reducing the fluid passage time is to employ a compact heat exchanger with a highly efficient heat transfer performance, a heat transfer length of only 21.5 mm, and a hydraulic diameter of 0.32 mm. A compact heat exchanger was used to build a dynamic generation setup for subcooled water, and 40 wt% of non-freezing liquid and tap water was used as the working fluid for heat exchange to generate subcooled water. The results show that the compact heat exchanger can achieve a greater subcooling degree (3.8 K) and longer duration (108 min). This study further explored the potential for dynamic ice making from deep, subcooled water and improved the overall structure of the compact heat exchanger used. The experimental setup is recommended based on the analysis of the results.

Investigation on the evolution of ice particles and ice slurry flow characteristics during subcooling release

For the subcooling method ice slurry generator, the characteristics of ice slurry during subcooling release have a significant impact on the efficiency and stability of the system. An object of this paper is to investigate the evolution and distribution of ice particles in supercooled water and the flow characteristics of ice slurry during subcooling release in horizontal straight pipe, based on computational fluid dynamics (CFD). Moreover, the population balance model (PBM) is adopted to research the growth, agglomeration and breakage of ice particles. The effects of velocity, subcooling degree and ice volume fraction at the inlet are investigated. In terms of pressure and velocity, the average error between numerical simulations and experimental results is approximately 10%. The results indicated that the evolution of ice particle diameters is significantly affected by the degree of subcooling, which contribute to the growth of ice particles. For each 1 K increase in supercooling degree, the particle diameter will increase by 50 µm. The temperature trend of water from the supercooled state to the phase equilibrium state is consistent with the evolution of ice particles. Besides, the ice volume fraction along the vertical axis at the outlet can be divided into low-concentration, high-concentration, and stable regions, which is significantly affected by the degree of supercooling. Close to the wall, the ice volume fraction drops sharply due to collisions between the wall and the particles. The velocity of the ice slurry is symmetrically distributed along the vertical axis.

Heat transfer and interfacial flow physics of microgravity flow boiling in single-side-heated rectangular channel with subcooled inlet conditions – Experiments onboard the International Space Station

This study is the culmination of a long-term collaborative effort between researchers from the Purdue University Boiling and Two-Phase Flow Laboratory (PU-BTPFL) and the NASA Glenn Research Center to investigate gravitational effects on flow boiling and flow condensation. The science and design concepts for this large-scale effort were initiated in 2011 and included several studies detailing various aspects of two-phase fluid physics in both Earth gravity and microgravity, culminating in construction of the large-scale experimental facility named “Flow Boiling and Condensation Experiment (FBCE)”. The experiment was launched to the International Space Station (ISS) in August 2021. Following the successful installation of FBCE, equipped with the Flow Boiling Module (FBM), onboard the ISS and completion of several safety checks, flow boiling experiments were performed for five months from February 2022 until July 2022. This resulted in a large flow boiling database covering broad ranges of operating parameters and heating configurations spanning several research objectives. This study investigates microgravity flow boiling of n-perfluorohexane with subcooled inlet in a single-side-heated rectangular channel of dimensions 114.6-mm heated length, 2.5-mm heated width, and 5.0-mm height. Key operating parameters investigated are mass velocity (199.90 – 3200.13 kg/m2s), inlet subcooling (0.10 – 45.76°C), and inlet pressure (113.30 – 164.29 kPa). Images and image sequences acquired via high-speed-video are presented to elucidate the interfacial flow physics. To analyze and explain the effects of various parameters in microgravity, heat transfer results are presented as flow boiling curves, streamwise profiles of wall temperature and heat transfer coefficient, and parametric trends of local and averaged heat transfer coefficient. Mass velocity and inlet subcooling significantly influenced most of the aforementioned aspects of flow boiling, whereas effects of inlet pressure were comparatively insignificant. Although the data and observed flow physics might be different, the parametric effects and trends in microgravity are similar to vertical upflow in Earth gravity. Some cases, especially low mass velocities, high heat fluxes, and large degrees of inlet subcooling, experienced temporally anomalous flow behaviors caused by two-phase flow instabilities manifesting as flow reversals and resulted in deviations in overall trends. Severe thermodynamic non-equilibrium is observed throughout the channel. Overall, FBCE's ISS experiments were successful for subcooled inlet with single-sided heating of rectangular channel, and the collected data well established the various effects on flow boiling physics in highly controlled long-term microgravity conditions.

Effect of wall subcooling on condensation in a steam-air mixture on a vertical tube

In this study, the effects of wall subcooling on the condensation heat transfer in steam-air mixture on a vertical tube was investigated. The effect of wall subcooling on condensation heat transfer was analyzed experimentally and using heat and mass transfer model. The experimental analysis indicated that wall subcooling effect varied depending on the degree of subcooling, i.e., the decrement of condensation heat transfer became less in low subcooling. In addition, the noncondensable gas fraction affected the wall subcooling effect. Such phenomenon was also found in the analysis using heat and mass transfer model. Through analysis using heat and mass transfer model, it was found that the accumulation of noncondensable gas and steam diffusion have competing effects. The former deteriorates and the latter enhances the condensation heat transfer with increasing wall subcooling. In low subcooling condition, both effects have higher rate of change, resulting in the different subcooling effect with that of high subcooling condition. We then developed an empirical correlation of condensation heat transfer over a wide range of wall subcooling; this predicted the experimental results reasonably well, particularly at low subcooling.

An experimental investigation of subcooled pool boiling on downward-facing surfaces with microchannels

Upward-facing pooling boiling enhancement employing micro-structure has been extensively studied, yet it is rarely applied in the cooling system in microgravity or In-Vessel Retention (IVR) in nuclear industry. The main challenge lies in effective removal of the vapor columns and replenishment of the fresh liquid without the aid of the buoyancy force. To solve the issue, additional pressure induced by microchannel is introduced to yield spontaneous bubble removal and liquid replenishment in the present work. Downward-facing pool boiling of subcooled FC-72 are conducted on Plain, Straight- and Expanding-Channeled Surfaces (PS, SCS and ECS) with visualization experiments. The superheat at the onset of nucleate boiling (ONB) on channeled surfaces was decreased about 3 K compared to that of the PS, and the maximum heat transfer coefficient (HTC) of the SCS and ECS were improved by 149 % and 217 %, respectively. Compared to PS, the channeled surfaces are more independent from saturated pressure, displaying high and relatively consistent heat transfer coefficients under various pressure conditions. Additionally, with the increasing subcooling degree, channeled surfaces showed greater enhancement in heat transfer performance. Proved by the bubble dynamics, the ECS enabled a directional movement and effective removal of vapor bubbles even though the buoyancy force cannot be utilized under downward-facing condition. The proposed enhancement mechanism attributed to separated vapor–liquid flow paths is independent on gravity and achieves a satisfied pool boiling performance, thus is expected to provide a promising solution for thermal management systems in aerospace and nuclear industry.

Experimental study on subcooled pool boiling heat transfer of sodium dodecyl sulfate surfactant solution

The pool boiling heat transfer coefficient (HTC) and critical heat flux (CHF) of sodium dodecyl sulfate (SDS) surfactant solution under different concentrations and subcooling conditions were experimentally studied. The concentration of SDS was in the range of 0 to 3000 ppm, and the subcooling degree was in the range of 2 to 10 °C. The heat transfer coefficient under different subcooling conditions was theoretically analyzed, and the real-time fluctuation of the wall temperature is discussed in detail. It is found that the subcooling has little effect on the natural convection of pool boiling but significantly impacts the nucleate boiling and that the phenomenon of temperature overshoot (TOS) is stronger in the SDS solution than that in de-ionized water. Furthermore, the origin and development process of TOS were deeply analyzed by showing the development process diagram. Meanwhile, the CHF phenomena were visualized and traced experimentally. The influence of different subcooling degrees and SDS concentrations on the CHF was deeply explored, and the development process diagram of the vapor film under CHF conditions was plotted. The results provide detailed theoretical support for the HTC and CHF of SDS solution under subcooling conditions.

Key factors controlling the kinetics of secondary hydrate formation in the porous media

Secondary hydrate formation (SHF) usually occurs in hydrate formation and decomposition experiments, and its manifestations behave differently in promoting gas consumption and water-hydrate conversion. In this work, in order to investigate the kinetic processes and controlling factors of the SHF in the porous media, we conduct a series of formation and dissociation experiments in a three-dimensional cubic hydrate simulator (CHS). Hydrate dissociation is accomplished by the depressurization method after the formation experiments. The experimental results reveal that a rapid growth rate of equal magnitude in the SHF stage follows a relatively slow hydrate growth in the process of hydrate formation. The occurrence of the SHF is accompanied by a sudden pressure drop and temperature rise, and a heterogeneous temperature distribution shows significant effects on hydrate formation behaviors in the vessel. It has been verified that the SHF is controlled by hydrate thermodynamic stability at a subcooling degree of −0.8 °C during the hydrate formation. The SHF is initiated by the collapse and crack of the local thin hydrate film, which results in an accelerated triggering of gas diffusion for further hydrate formation. In the dissociation experiments, the slight temperature increase caused by the SHF can promote the hydrate dissociation rate. The subcooling degree in the hydrate formation can also be suitable for predicting SHF in the dissociation experiment. It is found that a rapid depressurization rate can effectively shorten the duration of SHF. These insights can be beneficial for the energy transition and natural gas hydrate extraction in the future.

Eco-friendly reversible photochromic epoxy resin microcapsules with strong acid and alkali resistance for energy storage via facile in-situ microencapsulation

Multi-functional microcapsule considered as a promising technique is attracting more and more attention with social development. This study successfully prepared four kinds of eco-friendly epoxy resin microcapsules both with reversible photochromic and energy storage properties via in-situ polymerization. For the microcapsules, methyl palmitate (MP), which dissolves photochromic dyes (spiropyran), was used as the core material, and four epoxy resins (EP) with various molecular structured were selected as the shell material. The properties such as thermal stability, energy storage, surface morphology and UV absorption technology of these designed microcapsules were systematically investigated by thermogravimetric (TGA), differential scanning calorimetry (DSC), scanning electron microscope (FE-SEM), and UV–vis spectrophotometer, etc. Allowing the selection of microcapsules with mentioned properties for specific environments. The results show that hydrogenated bisphenol A epoxy resin (CYDH-3000) microcapsules possessed lower activation energy and higher UV absorption. The thermal stability of bisphenol A epoxy resin (E51) microcapsules was as high as 328.4 °C. While Bisphenol F epoxy resin (NP170) microcapsules have the highest energy storage efficiency (E = 91.45 %). The lower subcooling degree of hain epoxy resin (MHR-050) microcapsules reached 20.3 °C, showing promising application prospects in the thermo-regulated field. Meanwhile, these four epoxy resin microcapsules can retain specific shapes and photochromic phenomena with little enthalpy change in 10 % H2SO4 and 10 % NaOH environments due to the corrosion resistance of epoxy resins, which establishing the technical foundation for its application in viscose spinning.

Experimental study on supercooled water release supercooled state for generating ice crystals

Ice blocking and low single-cycle ice production rate are the two major deficiencies of supercooled water ice making. A lot of research has been done by domestic and foreign scholars on controlling ice blocking. The ice production rate of a single cycle is theoretically proportional to the degree of subcooling. In fact, the subcooling release also affects the ice slurry concentration. To improve the ice slurry concentration, this paper investigates the factors of affecting the degree of subcooling release, including three factors: ultrasonic power, subcooling degree, and the residence time of subcooled water in the ice slurry generator, and experiments with a way to produce high concentration ice slurry. The results showed that the greater the degree of subcooling, the deeper the degree of subcooling release; the ultrasonic power and the residence time of subcooled water in the ice slurry generator had little effect on the degree of subcooling release. The average ice content of the ice slurry reached 30.5% after 8 min of continuous ice production in the experiment.

Experimental study on heat transfer characteristics of steam underwater direct-contact condensation

Introduction: The direct-contact condensation (DCC) of steam under water injection is the basic thermodynamic process of the bubble deaerator. In order to understand the complex coupling behavior of strong turbulence and fast phase-change heat transfer involved in the process.Methods: This study uses a visualized method and convective heat transfer model.Results: Since the contact area is affected by steam injection flow and sub-cooled degree is affected simultaneously, the trend of the condensation heat-transfer coefficient depends on the degree of their respective effects under each condition, and the maximum variation of the coefficient exceeds 104 W/m2.°C. Moreover, they still effect the period of steam plume, and the maximum variation of the period was beyond 80 ms.Discussion: Calculated the average condensation heat transfer coefficient and then produces the variation law of heat transfer coefficient under various conditions in one steam plume evolution period.

Program development for mass flowrate distribution optimization in the nuclear power plant

For the compact PWR with cartridge fuel component, the application of flow partition design can significantly flatten the core outlet temperature distribution, improve the outlet subcooling, or reduce the coolant flow, to make more effective use of the coolant and greatly improve the core design performance. However, the three-dimensional power distribution shape of the core will change greatly during its lifetime, and it is difficult to reasonably distribute the flow. In this study, the closed channel core flow partition and water density feedback program is designed by studying the basis of flow partition and designing an optimization method that considers various factors. The optimization search of core flow distribution is carried out to maximize the subcooling degree at the component outlet and flatten the core outlet temperature distribution. In addition, the minimum coolant mass flow and the maximum outlet temperature are estimated under the given core thermal power, three-dimensional distribution and minimum outlet subcooling.

Experimental study of electronic expansion valve opening on the performance of electric vehicle heat pump system at different compressor speeds

The good matching of the electronic expansion valve (EXV) opening and compressor speed in an electric vehicle heat pump air conditioning system is significant for improving the whole system performance. The matching of EXV opening and compressor speed and its effect on system performance was experimentally investigated and analyzed. The results showed that at various compressor speeds, the condensing pressure increased and the evaporating pressure decreased to a small extent with the decrease of EXV opening, thus increasing the pressure ratio. The condensing pressure had a jump phenomenon at a specific EXV opening, and the corresponding jump opening was different at each compressor speed. The COP reached the optimal values of 2.4, 2.6, and 2.1 at the EXV opening of 110, 100, 90 steps, respectively, due to the synergistic effect between compressor speed and EXV opening. Moreover, it was found that the changing trend of COP was consistent with that of condenser outlet temperature. The compressor speed and EXV opening had a more significant impact on condensing pressure than evaporating pressure. The influence of compressor speed on each performance parameter was greater than that of EXV opening, while the EXV opening significantly influenced the subcooling degree. Under different compressor speed conditions, changing the single adjustment step of EXV according to the influence of EXV on each parameter can realize the rapid and accurate control of the heat pump system performance, which can provide a reference for the formulation of relevant control strategies and the reduction of the system energy consumption.

Study on preparation and thermal properties of new inorganic eutectic binary composite phase change materials.

It is important to improve phase change materials (PCMs) with appropriate temperature and excessive latent heat to accelerate the application of latent heat energy storage technology in solar energy storage systems. In this paper, the eutectic salt of NH4Al(SO4)2·12H2O (AASD) and MgSO4·7H2O (MSH) was prepared and the performance was studied. The DSC results show that the optimum content of AASD in the binary eutectic salt is 55 wt%, the melting point was 76.4 °C, and the latent heat is up to 189.4 J g-1, which is suitable for solar power storage systems. Four nucleating agents (KAl(SO4)2·12H2O, MgCl2·6H2O, CaCl2·2H2O, CaF2) and two thickening agents (sodium alginate, soluble starch) are added to the mixture in varying proportions to improve its supercooling. The best combination system was 2.0 wt% KAl(SO4)2·12H2O/1.0 wt% sodium alginate with a supercooling degree of 24.3 °C. After thermal cycling tests, the best formulation of the AASD-MSH eutectic salt phase change material was determined to be 1.0 wt% CaCl2·2H2O/1.0 wt% soluble starch. The latent heat was 176.4 J g-1 and the melting point was 76.3 °C. The supercooling was still lower than 30 °C after 50 thermal cycles, which served as a benchmark for the next investigation.

Operator actions evaluation for MSGTR accident mitigation

The safety assessment of nuclear power plants for multiple failure accidents such as Multiple Steam Generator Tube Rupture (MSGTR) and the verification of the suitability of accident management measures have gained prominence in the nuclear engineering field after the Fukushima accident. Compared to Steam Generator Tube Rupture (SGTR), the MSGTR accident progresses rapidly due to the high break flow rate, and thus appropriate operator actions are essential to prevent the release of radioactive materials into the environment. It is crucial for the operators to identify the MSGTR early and perform each operator action within the allotted time in order to mitigate the accident successfully. In this study, a reference analysis model for MSGTR is developed, differences from SGTR are analyzed, and the required time for major operator actions are derived through various sensitivity analyses. In conclusion, the operator can confirm that there is a high possibility of MSGTR occurring through the changes in the subcooling degree at the hot-leg and the Pressurizer (PZR) water level. The appropriate time to be initiated for major operator actions are as follows: (1) Reactor Coolant Pump (RCP) manual trip within 10 min after the reactor trip, (2) Reactor Coolant System (RCS) temporary-cooldown operation through Main Steam Isolation Bypass Valve (MSIBV) and Steam Bypass Control System (SBCS) within 15 min after the reactor trip, (3) PZR spray operation – RCS temporary-cooldown operation in progress, and (4) RCS controlled-cooldown operation using Atmospheric Dump Valve (ADV) within 15 min after the end of the RCS temporary-cooldown operation. Additional possible action is the restart of RCP, but the execution of this action depends on the operator’s judgment. The results of this investigation have significant implications which can be utilized for improving the accident mitigation strategies and the operator training for MSGTR.

Investigation of the integrated fuel cell, battery, and heat pump energy systems

The integration of an advanced fuel cell battery electric vehicle was proposed to extend the driving range, increase the overall energy efficiency, and produce an additional cabin space heating service for on-board vehicle application in winter. The adopted heat pump absorbs heat from the fuel cell and battery during charging and discharging scenarios at the heat pump's cold end and transfers it to the cabin supply air for cabin heating. A numerical model for the proposed system was developed and validated by published experimental results. A comprehensive evaluation of the influence of fuel cell output current, battery discharging rate, ambient air temperature, evaporator coolant inlet temperature and refrigerant subcooling degree on the Coefficient of Performance (COP), outlet air temperature, and Equivalent Effective Battery Capacity (EEBC) were conducted. In addition, in this study, a detailed comparison of proposed system to previously published existing systems was carried out in terms of COP, heating capacity, EEBC, and operating price in order to examine the advantages of the proposed system. Results showed that the proposed system was not sensitive to ambient air temperature, but evaporator coolant inlet temperature had the greatest impact on COP, compared to other defined independent variables. A maximum COP of 6.07 could be observed when the evaporator coolant inlet temperature was 32 °C. Although increasing fuel cell output current and battery discharging rate could increase heating capacity, it would decrease outlet air temperature as well. The price of a single charging cycle for running for a 4-hour period was estimated. Compared to conventional Electric Vehicle Air Source Heat Pump (EVASHP) system and Positive Temperature Coefficient (PTC) heaters, the proposed system has the lowest cost which was £10.40per charging cycle while the others are £13.8 and £19.2 respectively when providing the same EEBC and heating capacity. Additionally, the payback period was 10,500 charging cycles. Overall, this study provides a potential solution for efficient cabin heating of electric vehicles in extremely cold weather with twice the EEBC of the existing EVASHP system and with 2.8 times of a PTC system both at −20 °C and the operating price is also 1/3 lower than the existing EVASHP. Meanwhile, the COP of the proposed system is 2.3 times that of the existing systems. It can sufficiently increase the driving range of electrical vehicles in winter and eliminate driving range anxiety.

Experimental Kinetic Evaluation of Carbon Dioxide Hydrate-Based Concentration for Grape, Pineapple, and Bitter Melon Juices.

Hydrate-based technology has emerged as a promising approach to address the industry's energy demands and product quality challenges in the food industry. Despite reported successes in the literature where higher dehydration ratios were achieved, technological problems like slow formation rates and poor process scale-up economics need to be addressed. Moreover, with little hydrate formation data available, the major focus is on the technology's ability to remove water content, but studies on the kinetics of hydrate formation are scarce. In the present work, the effects of varying grape/pineapple/bitter melon juice water cuts (88.5 to 97.4 ± 2.53 wt %) on the formation kinetics of carbon dioxide (CO2) hydrates were investigated. Such information can provide insight into the possibile commercialization of the hydrate-based technology. The reported experimental data were determined using the isochoric pressure-search method in a high-pressure reactor at a target initial temperature from 274.15 to 276.15 K and varying initial pressures. Kinetic parameters were calculated using the relative kinetic models proposed in the literature. Lower relative values of investigated kinetic parameters and longer induction times were obtained at lower juice water cuts and lower degrees of subcooling. Despite observed inhibition effects, the study provides useful experimental and modeled kinetic data for filling the knowledge gap in understanding the controlling mechanism of CO2 hydrate formation. Therefore, it is believed that the reported findings may highlight some important practical aspects related to CO2 hydrate technology as an alternative juice concentration process.

Heat transfer coefficient of boiling flows in minichannels with a semicircular cross-section

The boiling heat transfer characteristics of vertically upward flows in small-diameter semicircular channels heated with hot water were experimentally investigated to characterize the influences of channel cross-section and thermal diffusivity of a heat transfer wall on heat transfer characteristics. The inlet subcooling degree was set to 10 K at a saturated temperature of 30 °C with a mass flux of 100–400 kg/(m2 s). Two types of hydraulic channels (stainless steel and copper) with diameters of 1.04 and 0.55 mm were used with refrigerant R-245fa. Moreover, the average heat transfer coefficient was evaluated considering the fin efficiency of the heat transfer wall. As a result, the heat transfer coefficient for high mass flux increased with the outlet quality owing to the nucleate boiling in the channel corners, causing a strong fluctuation of the gas–liquid interface. For the 0.55-mm channel, as the liquid film was thicker in the corners than in other regions due to surface tension, the increase in thermal resistance at the corners promoted nucleate boiling. A new correlation equation was derived to express the heat transfer coefficient for semicircular minichannels considering the fin efficiency; the mean absolute error between the experimental data and calculated values was 11.3 %.

Frequency domain analysis of density wave oscillations for two-phase flow in a vertical tube

In order to investigate the boundary characteristics of density wave oscillations in nuclear reactor steam generators and boiler water-cooled wall tubes, a frequency domain method numerical calculation model applicable to different geometrical structures and operating parameters was developed. In order to make the results of the model more accurate, the state of the fluid at the outlet of the heated pipe and the time delay were fully considered on the basis of the traditional model. Depending on the physical properties of the fluid at the outlet of the heated pipe, the model calculation area was divided into single-phase, two-phase and superheated zones. Small-perturbation linearization and Laplace transform were performed on the conservation equations in the three regions. The Nyquist stability criterion was used in matlab software to judge the stability of the flow, and by varying the working parameters the flow instability boundary can be calculated. For a more in-depth study of density wave oscillations in heated pipes, the unstable boundary characteristics were analysed computationally for different influences such as pressure, mass flow rate, inlet subcooling degree and pipe geometry on the basis of model validation. In addition, the Nyquist function was used in Matlab to plot the Nyquist curve for different influencing factors and to analyse the stability margins for specific operating conditions. The unstable boundary characteristic diagram was obtained when the pressure was 5–10 MPa and the mass flux was 300–1300 kg·m−2·s−1. The calculated data was collated and mass flow rate and heat load boundary diagrams were plotted, which could visualise the boundary characteristics of the density wave oscillations under different operating conditions. In order to make the calculation results more applicable, Nsub and Npch were used to construct the dimensionless unstable boundary characteristic diagram. Results showed that the influence of inlet subcooling degree on flow stability is non-monotonic. In addition, the effects of factors such as pressure, mass flow rate and inlet throttling coefficient are monotonic.