Two-phase Natural Circulation Loop Research Articles

Research on characteristic of flow instability in a two-phase natural circulation loop with parallel once-through steam generators

When there are two sets of parallel once through-steam generators(OTSG) in the natural circulation loop, the thermal-hydraulic characteristics are complicated. Flow instability often occurs in such loops. In order to alleviate the harm caused by flow instability and enhance the natural circulation capacity, it is necessary to study the characteristics of flow instability in depth. In this paper, a thermal-hydraulic computational model is developed for a two-phase natural circulation loop with two parallel OTSGs based on the RELAP5 program. And the model is validated. The thermal-hydraulic parameters of the loop were calculated for different decay heat conditions. The flow instability of the loop is found to be characterized by a clear division into three zones in the decay heat interval. The concept of “load-flow ratio” is proposed to analyze the critical point of flow instability under different operating conditions. The reason for the flow instability zones of the loop is explained. The conclusions of the relevant research can provide a basis for an in-depth study of the characteristics of flow instability and the development of operational schemes to mitigate flow instability.

Analysis of coupled two-phase natural circulation loops by developing HEM and DFM based numerical models

The coupled two-phase natural circulation loop based passive systems find numerous applications, particularly in nuclear reactors where safety is paramount. However, the loops coupling, system conditions (specific to the reactor type), and inherent coupled loops feedback make their dynamics more complex. Hence, the models used should be capable of predicting such intricate coupled dynamics for precise forecasting of passive systems performance. With this objective, the present analyses deliberate the transients of such coupled two-phase loops linked to coupling configurations and operating conditions. Generalized two-phase models based on Homogeneous Equilibrium (HEM) and Drift Flux (DFM) with flashing, wall and fluid conduction, water-steam property formulation, etc., are developed and employed. Validation studies substantiate the prediction capability of these models for various cooler-heater orientations. Subsequently, predicted the start-up transients of vertically and horizontally coupled two-phase loops. The analyses revealed that coupling configurations, heater-cooler orientations and system conditions significantly influence the coupled loops. Further, the influences of various geometrical and operating parameters on the dynamics of coupled loops are investigated. It elucidated the differences in the parameter's effect on single-phase and two-phase coupled loops. The study also identified the vital parameters strongly influencing the coupled loops based on sensitivity analysis. The models and findings will be applied to evaluate the performance of coupled loop-based passive safety systems in nuclear reactors.

Analysis of the heat-carrying performance of natural circulation loop using modified RELAP5/MOD3.4

Since the successful manufacture of the separate heat pipe (also known as Two-phase Loop Thermosyphon or Natural Circulation Loop), it has been used in many applications, especially for waste heat recovery/remove and cooling systems. We use the modified system analysis program RELAP5 to study the medium-sized separated heat pipe setup MSL2 built by Seok-Ho Rhi. To better calculate heat pipe performance under low pressure, we have improved a series of models, including the bubble nucleation model, the heat flux partition model, the interfacial heat transfer, the drift flux model, the wall heat transfer model, etc. Then we use the low-pressure subcooled boiling experiment to verify the modified code. The validation results show that the modified code accurately captures the subcooling boiling phenomenon under low pressure and low flow conditions. We use the modified code to investigate the factors influencing heat-carrying performance and analyze their action mechanisms. The results show that heating power, volumetric filling rate, and cooling water temperature influence heat-carrying performance. Additionally, we analyze the heat pipe operating parameters over time to obtain the characteristics of the heat pipe at various stages.

Effect of geometrical and operating parameters on performance of a rectangular two-phase natural circulation loop

Natural Circulation Loops (NCLs) find application in a wide range of engineering devices and it is often necessary to predict and ensure the performance of such loops under various operating conditions. In this paper analytical expressions are presented in non-dimensional form for steady-state mass flow rate in typical rectangular, two-phase natural circulation loops having Vertical-Heater Vertical-Cooler (VHVC) and Vertical-Heater Horizontal-Cooler (VHHC) configurations. The normalized loop mass flow rates are expressed in terms of several geometrical and operating variables such as, normalized heat input, density ratio, volume/diameter of heater and cooler sections, aspect ratio of loop, inlet subcooling, fractional length of heater and cooler sections, two-phase friction multiplier and the loss coefficient at heater inlet. The analytical mass flow rate expressions are first validated using published data for various loops and shown to reasonably capture the gravity and friction dominated regimes in two-phase pipe flow. Parametric studies are then conducted to elucidate the effect of key variables on steady-state loop mass flow rates. For the chosen geometrical configuration of the loops, the possibility of existence of multiple steady-state solutions is checked for the entire range of values used in this study for each variable. The discussions in this paper are limited to explaining the influence of individual variables by comparing with a base case analysis.

Analytical study of flow instability behaviour in a boiling two-phase natural circulation loop under low quality conditions

Abstract Analytical investigations have been carried out to study the flow instability behaviour in a boiling two-phase natural circulation loop under low quality conditions. For this purpose, the computer code TINFLO-S has been developed. The code solves the conservation equations of mass, momentum and energy and equation of state for homogeneous equilibrium two-phase flow using linear analytical technique. The results of the code have been validated with the experimental data of the loop for both the steady state and stability. The study reveals that the stability behaviour of low quality flow oscillations is different from that of the high quality flow oscillations. The instability reduces with increase in power and throttling at the inlet of the heater. The instability first increases and then reduces with increase in pressure at any subcooling. The effects of diameter of riser pipe, heater and the height of the riser on this instability are also investigated.

Effect of Geometrical and Operating Parameters on Performance of a Rectangular Two-Phase Natural Circulation Loop

Effect of Geometrical and Operating Parameters on Performance of a Rectangular Two-Phase Natural Circulation Loop

Experimental investigation of two-phase natural circulation loop as passive containment cooling system

In this study, we experimentally investigate of a two-phase natural circulation loop that functions as a passive containment cooling system (PCCS). The experimental apparatus comprises two loops: a hot loop, for simulating containment under severe accidents, and a natural circulation loop, for simulating the PCCS. The experiment is conducted by controlling the pressure and inlet temperature of the hot loop in the range of 0.59–0.69 MPa (abs) and 119.6–158.8 °C, respectively. The heat balance of the hot loop is established and compared with a natural circulation loop to assess the thermal reliability of the experimental apparatus, and an additional system is installed to measure the vapor mass flow rate. Furthermore, the thermal–hydraulic characteristics are considered in terms of a temperature, mass flow rate, heat transfer coefficient (HTC), etc. The flow rate of the natural circulation loop is induced primarily by flashing, and a distortion is observed in the local HTC because of the fully develop as well as subcooled boiling. As a result, we present the amount of heat capacity that the PCCS can passively remove according to the experimental conditions and compared the heat transfer performance using Chen's and Dittus-Boelter correlation.

Effect of heater and condenser orientations on fluid flow behaviour in a two-phase natural circulation loop

A Numerical investigation on the influence of heater and condenser orientation and its location on steady state performance of a Two Phase Natural Circulation Loop (TPNCL) is presented. In the present study, four possible orientations are considered in TPNCL namely, Vertical Heater and Vertical Condenser (VHVC), Vertical Heater and Horizontal Condenser (VHHC), Horizontal Heater and Horizontal Condenser (HHHC) and Horizontal Heater and Vertical Condenser (HHVC). An in house MATLAB code is developed to solve the systems of equations. Homogeneous equilibrium model (HEM) is used in two phase regions. It is observed that steady state mass flow rate of the loop is significantly influenced by the developed pressure gradient in the riser and downcomer sections, as well as available vapour quality in the riser section. Moreover, it is also noticed that condenser position and orientation significantly effects the thermo-hydraulic characteristic of loop compared with heater orientation.

Using organic fluids in natural circulation loop systems for absorbing of heat from low temperature renewable energy sources

The main objective of this paper is to investigate the usage of organic fluids as the working fluid in the two-phase natural circulation system. Also, this paper discusses the effects of fluid material properties on the performance of the system. For this purpose, five working fluids are selected, include water, R113, R11, Cyclohexane, and R245fa. The mathematical simulation of the system is done by applying a two-phase flow model. The mass flow rate of the two-phase natural circulation for five working fluids are shown in different heating power and sub-cooling conditions. To analyze the transient state behavior of the two-phase natural circulation loop, the stability maps of the system for five working fluids are sketched. According to obtained results, using organic fluids instead of ordinary water can increase both the stability and capability of removal heat. Furthermore, the second law of thermodynamic is applied to calculate the amount of entropy generation, and consequently to study the thermal-hydraulic performance of the different working fluid used in the loop.

Cryogenic two-phase natural circulation loop

This paper presents the application of cryogenic natural circulation loop for engine cooling of launch vehicles, and the experimental measurements of the natural circulation system. The experimental results indicate that the circulation flow rate increases with an increase in heat input and a decrease in system pressure, on varying them. The study includes the temperature and pressure profiles of the natural circulation loop. The steady-state flow rate calculation was developed with two-phase models: the homogeneous equilibrium model (HEM) and thermal equilibrium drift-flux model (TEDFM). The TEDFM revealed a 5% error in the experimental circulation flow rate, and it is agreeable with the experimental results. The small-scale experimental setup verified the two-phase flow pattern in the cryogenic natural circulation loop. A fully liquid flow was observed in the downward section. The horizontal section had a bubbly flow, where the speed of the gas and liquid were similar. The slug and churn flows were present in the vertically upward section, where gas velocities were faster than liquid. Both HEM and TEDFM can be applied in the horizontal heated section. However, it is more appropriate to apply TEDFM in the vertically upward section during the pressure drop calculation for the cryogenic natural circulation loop.

Experimental investigation on the oscillation suppression in a R134a two-phase natural circulation close loop singly connected to an accumulator

Having the advantage of durability (simple in structure without mechanical pump), two-phase natural circulation (TPNC) loops were widely used, but still limited by its inherent instability that may lead to evaporator dry out (boiling crisis). Till now, the mechanism of the TPNC instability are not fully understood. In our previous experiment in a TPNC close loop with a singly connected accumulator (Wen et al., 2018) [1], two types of the density wave oscillations (DWO) were identified by characteristic frequencies; and the stable region between the two types of DWO can be expanded by reducing the subcooling. In the present research, we found an effective way to suppress the two types of the oscillation by increasing the resistance coefficient of connection pipe between the accumulator and the loop, or even eliminate the oscillation simply by shutting off the pipe connection. We inferred that the exchange of fluid between two compressible volumes (the accumulator and the two-phase section of the loop in the present case) is a necessary condition for the two types of the DWO. It was found that, the type 2 density wave oscillation would lead to the evaporator dry out, causing about 25% deterioration of the loop’s heat dissipation capacity as compared with the stable loop when the oscillation was suppressed. Also, scaling analysis has been performed.

Experimental study of hydrodynamics and heat transfer in two-phase natural circulation loop with reference passive cooling systems of nuclear power plants

In spite of long year practical use of natural circulation systems and not less long period of study working characteristics of these systems the problem of reliable predicting calculations of natural circulation loops (NCL) with boiling coolant at low reduced pressures is not likely can be considered as being solved. At the same time the interest to solution of this problem arises today, because low pressure natural circulation loops are considered as main type of post accident passive cooling systems for nuclear power plants. The main difficulties for calculations are concerned with determining friction pressure losses, which are strongly depended on two-phase flow pattern. At low reduced pressures large difference between liquid and vapour (gas) specific volumes predetermines the strong change of flow pattern even at small change of mass flow quality. An attempts to calculate local two-phase flow parameters (void fraction, circulation velocity) in the wide range of mass flow qualities without considering the change of flow patterns often lead to large deviations of calculated values from the results of experimental measurements. Another specific feature of two-phase low pressure NCL is hydrodynamic flow instability with circulation velocity pulsations of high amplitude and the occurrence of reverse flows. This also presents problems in developing calculation method. In present paper a method for calculating a low-pressure NCL has been developed, in which local two-phase flow parameters (void fraction, the phases velocities, pressure) are calculated using a modified homogeneous model with taking into account the distribution factor and the phases slip and a model of an annular-dispersed flow with considering droplets entrainment and deposition. A modified homogeneous model was applied for description quasihomogeneous flow regimes. At high values of void fraction annular-dispersed flow model was used. Recommendations for change from one model to another in practical calculations have been formulated and verified. The proposed calculation method has been verified by the comparison the calculated thermo-hydraulic characteristics of laboratory natural circulation loop with the experimental data. The experiments have been carried out for boiling in the loop of three different liquids – water, ethanol and perfluorohexane (FC-72) at atmospheric pressure. The comparison of calculated and experimental results showed their good agreement.

Experimental and Calculated Investigation of a Natural Circulation Loop’s Thermal-Hydraulic Characteristics

The results from experimental investigation into hydrodynamics and heat transfer in a two-phase natural circulation loop (NCL) under atmospheric pressure are presented. The experiments were carried out for liquids having essentially different properties: water, ethanol, and perfluorohexane C6F14 (the product trademark is FC-72). The circulation velocity in the NCL is not known in advance but is a complex function of the specified parameters (heat flux and liquid temperature at the heated section inlet) and of the two-phase flow internal characteristics. The liquid temperatures at the heated section inlet, the wall temperature over the section height, and also the circulation velocity were measured in the experiments at a specified heat flux; in addition, the two-phase flow at the loop riser leg outlet was filmed on video. The experiments and analysis have shown that flow hydrodynamic instability (circulation velocity pulsations) is really unavoidable in a two-phase NCL. Hydrodynamic instability with a high circulation velocity amplitude and with the occurrence of backward flows is typical for regimes involving significant liquid subcooling values at the heated section inlet and for NCLs containing an extended part with single-phase convection. This instability, which is characteristic for experiments with water, is due to the displacement of the boiling incipience section over the section height; the instability also persists at small subcooling values but with a low pulsation amplitude. Under the developed saturated liquid nucleate boiling conditions (at high heat flux values), the circulation velocity and wall temperature pulsations have small amplitudes, and the flow can be regarded as stable. In the experiments with perfluorohexane, the smallest wall temperature and circulation velocity pulsations were pointed out, which is attributed to a relatively high value of reduced pressure. In the experiments with ethanol, instability occurs in the developed nucleate boiling region (q > 35 kW/m2); this instability is caused by periodically alternating two-phase flow structure (regime). A procedure for calculating a low-pressure NCL is developed, in which the two-phase flow’s local parameters (void fraction, phase velocities, and pressure) are calculated according to a modified homogeneous model (taking into account the phase distribution factor and phases slip) and a dispersed-annular flow model taking into account the droplet entrainment and deposition phenomena. A comparison of the NCL calculation results with the experimental data obtained for three different liquids has shown that they are in good agreement with each other.

Two-phase natural circulation loop behaviour at atmospheric and subatmospheric conditions

This paper aims to present the steady-state behaviour of two-phase natural circulation loop at atmospheric and sub-atmospheric conditions. One-dimensional numerical approach is adopted to evaluate various system parameters, with special emphasis on spatial variation of thermo-physical properties and flashing. Homogeneous equilibrium model is applied for two-phase flows. An in-house code is developed in MATLAB to solve numerical model iteratively. It is observed that consideration of spatial variation of thermo-physical properties can precisely predict the loop behaviour. The evaluated results are validated with the open literature and reasonably good agreement is observed. The heater inlet temperature, inlet pressure and heat flux are found to have significant influence on spatial variation of pressure, temperature and enthalpy. As system pressure decreases from atmospheric to sub-atmospheric (1–0.8 atm), it is observed that the sub-atmospheric loop gives a higher mass flow rate compared to atmospheric loop at lower heat fluxes. However, as the heat flux increases in the sub-atmospheric loop, the mass flow rate is reduced due to increased drag force in the loop.

Experimental research of a two-phase nitrogen natural circulation loop

Two-phase heat and mass transfer in nitrogen has been experimentally investigated in a natural circulation loop. The experiments were realized in steady-state conditions near atmospheric pressure with a 10 mm inner diameter vertical copper tube, uniformly heated over roughly 1 m in length. The vapor and total mass flow rates and the wall temperature were measured as a function of the tube heat flux density. The investigated ranges-limits are 40 g/s, 25% and 30 kW/m2 for the total mass flow rate, the vapor quality and the heat flux density respectively. Transition from single phase natural convection to nucleate boiling flow is evidenced by wall temperature measurements at five different locations along the heated tube. The measurements are compared to the numerical computations of the total mass flow rate that is simulated by a set of conservation equations based on a simple separated flow model. The model catches most of the features of the mass flow rate evolution as a function of the heat flux density. In addition, the wall temperature measurements are used to determine the heat transfer coefficients that are compared with known correlations in both single phase natural convection and boiling flow. A majority of the data falls within ±20% of the predicted values given by the correlations.

Assessment of RELAP5 matrix solvers for a two-phase natural circulation loop

The heat transfer mechanism of natural convection has been extensively studied as a passive heat removal system of new nuclear power plants. Considering this aspect, the main objective of this study is to present an assessment of RELAP5 linear-equation solver under a transient two-fluid model for a two-phase natural circulation loop (NCL). For this assessment, three different approaches of linear-equation solvers for the hydrodynamic model are presented: the sparse matrix solver based on the Lower-Upper (LU) decomposition, the Border-Profile Lower Upper (BPLU) solver and the iterative method named Generalized Minimal Residual Method (GMRES). For comparison purposes, an experimental natural circulation loop made of glass tubes and using water as working fluid is analyzed. The onset of nucleate boiling observed during the experiment was predicted by all RELAP5 solvers as well as the representation of flow oscillations along the loop. Furthermore, it was noticed that the choice of the solver algorithm has a strong influence on the prediction of the two-phase natural circulation phenomena, since different wavelengths and amplitudes of flow instabilities were obtained for each approach.

Study on flow characteristics in an open two-phase natural circulation loop

Flow characteristics are relevant to the safe operation of natural circulation systems. Experimental research on flow characteristics in an open natural circulation loop was conducted. It was shown that flashing and geysering are the two fundamental phenomena influencing the flow characteristics; in addition, flashing and geysering interact reciprocally: geysering weakens the impact of flashing on the flow, while flashing can suppress geysering. Six circulation modes were identified based on the weight of flashing and geysering, as well as typical inlet flowrate curve and experimental observations. As the heating power varies, so do the weights of flashing and geysering, which result in variable flow behavior and the evolution of circulation modes. Based on the theoretical analysis and the calculation of average circulation flowrate by taking into account the effects of subcooling, flashing and heat dissipation, suggestions for the operating and improvement of the system were proposed.

Steady-State Performance of a Rectangular Natural Circulation Loop With Differentially Heated Parallel Channels

Natural circulation loop (NCL) transfers thermal energy without using any external power. As with phase change, one can expect a higher rate of heat transfer and a greater change in density, NCL with a phase change of the circulating fluid is a more effective energy transfer device. Though in many of the practical NCLs there are multiple heating risers, the characteristics of NCLs with parallel boiling risers have not been investigated in detail. In the present work, the steady-state behavior of a two-phase NCL with two parallel boiling risers for water as the working fluid has been investigated. Emphasis has been given to the performance of the loop when the risers are differentially heated. Effect of different parameters on the loop circulation rate and energy transport for both equally and differentially heated conditions has been thoroughly examined and compared to the performance of a single-riser loop under equivalent working condition.

Improving performance of two-phase natural circulation loops by reducing of entropy generation

This paper aims to investigate the effects of various parameters on stability behavior and entropy generation through a two-phase natural circulation loop. Two-phase natural circulation systems have low driving head and, consequently, low heat removal capability. To have a higher thermodynamic efficiency, in addition to the stability analysis, minimization of entropy generation by loop should be taken into account in the design of these systems. In the present study, to investigate the stability behavior, the non-linear method (known as the direct solution method or time domain method) which is able to simulate the uniform and non-uniform diameter loops, was applied. To best calculate entropy generation rates, the governing equations of the entropy generation were solved analytically. The effects of various parameters such as operating conditions and geometrical dimensions on the stability behavior and the entropy generation in the two-phase natural circulation loop were then analyzed.

Steady state boiling crisis in a helium vertically heated natural circulation loop – Part 2: Friction pressure drop lessening

Experiments were conducted on a 2-m high two-phase helium natural circulation loop operating at 4.2K and 1atm. Two heated sections with different internal diameter (10 and 6mm) were tested. The power applied on the heated section wall was controlled in increasing and decreasing sequences, and temperature along the section, mass flow rate and pressure drop evolutions were registered. The post-CHF regime was studied watching simultaneously the evolution of boiling crisis onset along the test section and the evolution of pressure drop and mass flow rate. A significant lessening of friction was observed simultaneous to the development of the post-CHF regime, accompanied by a mass flow rate increase, which lets suppose that the vapor film in the film boiling regime acts as a lubricant. A model was created based on this idea and on heat transfer considerations. The predictions by this model are satisfactory for the low quality post-CHF regime.