Vertical Narrow Rectangular Channel Research Articles

Neural network based two-phase flow classification in a vertical narrow rectangular channel

Flow Regimes in a vertical narrow rectangular channel of cross-section 20×1cm2 are investigated up-to wispy annular flow using a dense test matrix and double sensor conductivity probe at section mid point. The data from the probe is used to calculate void fraction, velocity of interfaces, and chord length of various flow structures. A five unit self organizing neural network is used to identify various flow regimes by using single point geometrical data of flow structures. Six separate flow regimes are found to exist. A new flow regime is identified and is called rolling-wispy flow. A discussion on boiling crisis is given regarding this flow regime. The resultant flow regime map is compared with various existing maps.

Experimental investigation on steam-water flow patterns in a vertical narrow rectangular channel utilizing conductance scanning measurement

In order to tackle the challenges in two-phase flow measurement in narrow rectangular channels, especially under pressurized conditions, a novel conductance scanning sensor, consisting of two ceramic-substrate printed circuit (CSPC) boards, has been developed based on the concept of wire mesh sensor. A leak-tight structure was also fabricated to avoid leakage at the penetrations of lead wires through the sensor housing. Measurements have been carried out in a narrow rectangular channel, with inner the cross-sectional dimension 70 mm × 2 mm, under pressure between 0.20 and 0.90 MPa. In the experiment, bubbly, churn and annular flows have been observed, except for slug flow. Flow pattern maps have been proposed for the steam-water two-phase flow. Comparing with those in the literature, it can be found that as the channel width/gap increases, the flow pattern transition boundaries shift towards the higher vapor superficial velocity direction. Additionally, the transition criteria for narrow rectangular channels were investigated by separately evaluating the critical void fraction and the superficial velocity calculation correlations. Results show that the critical void fraction of bubbly to churn flow and churn to annular flow can be approximately estimated as 0.2 and 0.75, respectively. However, the existing correlations for superficial velocity calculation are not suitable for the steam-water flows with a void fraction less than 0.5.

Study of flow pattern transition criterion and heat transfer characteristics in vertical rectangular narrow channel for steam heating

This study clarifies the heat transfer characteristics of flow boiling in a narrow vertical rectangular channel at each stage and optimizes the performance of a heat exchanger. The flow pattern and local heat transfer characteristics in a narrow vertical rectangular channel of steam heating with dimensions of 1400 × 250 × 2.75 mm (l × w × s) were experimentally investigated using deionized water as the working medium. Based on the visualization data, the mechanical model and empirical formulas in previous literature, a preliminary derivation of the flow pattern transition criterion for steam heating in a vertical rectangular narrow channel was obtained. The effects of the inlet temperature, mass flux, and heating power on the local heat transfer characteristics were analyzed in conjunction with the flow patterns. Based on the parametric analysis of the experimental data, a new flow boiling heat transfer correlation formula was obtained and applied to the experimental conditions. The results showed that an increase in the inlet temperature had a positive effect on the nucleate boiling section, whereas the effect on the local heat transfer coefficient was minimal after entering the churn flow phases. The increase in mass flux decreased the local heat transfer coefficient in the nuclear state boiling stage and enhanced the local heat transfer coefficient in the evaporation zone of the liquid film flow. The peak of the local heat transfer coefficient tended to shift to the left as the heating power increased. The proposed heat transfer correlation equation could well predict the local heat transfer coefficient of flow boiling in a vertical rectangular narrow channel heated by steam under these experimental conditions.

Experimental investigation on heat transfer characteristics of upward flow boiling in a vertical narrow rectangular channel

ABSTRACT Heat transfer experiment on water flow boiling was carried out in the upward direction under atmospheric pressure through a narrow rectangular channel heated on one-side by vapor having a gap of 2.75 mm, a width of 250 mm, and length of 1400 mm. The heat transfer coefficient and thermal hydraulic thresholds of the flow boiling in forced convective flow, such as the onset of nucleate boiling (ONB) and onset of fully developed nucleate boiling (OFDB) were investigated. The experiment was performed over a wide range of inlet temperature (75–95°C), mass flow rate (2–9 g/m), and heat fluxes (5–16 kW/m2). The effects of the parameters on the heat transfer coefficients have been discussed in detail. A series of ONB, FDB and heat transfer correlations were evaluated using the experimental data, and most of the correlations did not adequately fit the experimental results. A modified Hong et al. correlation and modified Zhu et al. correlation were used to predict the wall superheat at ONB and FDB respectively. The average errors (AEs) of the two correlations were 2.36% and −0.68%, and the root mean square errors (RMSEs) of them were both 14.00%. A modified Li and Wu correlation was used to predict heat transfer coefficients with the average error (AE) of 1.46% and the root mean square errors (RMSE) of 11.88%.

The experimental research on subcooled flow boiling in vertical narrow rectangular channels with relatively Wide-Ranged parameters

A series of tests are performed for subcooled boiling heat transfer characteristics of water in narrow rectangular channels. The channel sizes are 50mm×3mm×1000mm, 60mm×3mm×1000mm and 50mm×2mm×1000mm. The test matrix covers a relative wide range of conditional parameters with a mass flux range of 2003000kg/m2∙s, a system pressure range of 0.25.5MPa and an inlet subcooling of 20100K. The images of bubble behaviors are caught by high-speed camera. The images analysis and sensitive study on each parameter of boiling incipience and subcooled boiling heat transfer characteristics are carried out. The effect of forced convection could not be neglected in wall temperature of ONB prediction. Subcooled boiling heat transfer capacity gets enhanced by bubble sliding. According to the force balance analysis and bubble behavior photograph, flow condition is the main factor to promote bubble departure. To consider the appearance of confined bubble in narrow rectangular channel, the particular confined aspect ratio number α∗ is proposed. By measuring α∗, the effect of geometric characteristic on subcooled boiling heat transfer could be known. The new correlation for ONB prediction and subcooled boiling heat transfer in narrow rectangular channel is developed within the deviation of ±20%. In particular, the ONB prediction correlation shows good performance in prediction based on the database from other literatures.

Two-phase interfacial structure development in vertical narrow rectangular channels

This study has experimentally investigated flow characteristics of two group bubbles in upward two-phase flows in a vertical narrow rectangular channel with 0.993 mm in gap, 40 mm in width, 2000 mm in height, and 1.94 mm in equivalent hydraulic diameter, Dh. The local measurements are performed at seven axial positions (z/Dh =52, 104, 155, 207, 415, 622, 829 (z: the axial distance from the inlet) using a high-speed camera. The database of the locally evolving parameters has been established by processing the images from the high-speed camera measurements. The measured parameters include the void fraction, interfacial area concentration (IAC), Sauter mean diameter and number density of group-1 and -2 bubbles, and gas-phase velocity of all bubbles. A total of 14 flow conditions are set at four different superficial liquid velocities ranging from 0.214 m/s to 2.08 m/s and different superficial gas velocities ranging from 0.0755 m/s to 1.70 m/s at the inlet. The observed flow regimes cover the bubbly, cap-bubbly, and slug flows. The measured void fraction ranges from 3.92 % to 42.6 %. The axially developing features of the two-phase flows are analyzed for the three flow regimes. Abrupt and gradual axial two-group bubble evolutions with intensive bubble coalescence at the flow regime transition are, respectively, observed at low and high superficial liquid velocity conditions. The newly-obtained experimental database has been used to evaluate the existing two-group interfacial area transport equation (IATE) and its corresponding source and sink models for the two-phase flow in the vertical narrow rectangular channel. Group-1 bubble IAC mean relative error of 21.1% and group-2 bubble IAC mean relative error of 33.4% have been obtained in the evaluations. The existing two-group IATE cannot satisfactorily predict the intensive bubble coalescence and the intragroup transport of the two-group bubbles at the transition from bubbly to slug flow in the vertical narrow rectangular channel. Therefore, it is necessary to improve the existing models of the intergroup and intragroup bubble interactions in the existing two-group IATE in the future.

Liquid film thickness of vertical upward annular flow in narrow rectangular channel

The interfacial shear stress and liquid film thickness are very important to establish the basic model of annular flow in practical applications. In this paper, the liquid film thickness of annular flow in a narrow vertical rectangular channel with a cross-sectional area of 70 mm × 2 mm was measured based on the PCB liquid film sensor. According to the experimental results, the average liquid film thickness, base film thickness and interfacial friction factor were calculated and analyzed. The experimental results showed that the liquid film thickness was affected by the gas phase velocity and the liquid phase velocity, but the effect of the gas phase velocity on the liquid film thickness decreased gradually with increasing gas phase velocity. Moreover, the interfacial friction factor was obvious affected by the liquid phase velocity at low gas phase velocity, but this influence gradually decreased with increasing gas phase velocity. Existing correlations for above parameters were summarized and evaluated with the current data, and there was a large deviation between the predicted results and the experimental results. Therefore, a new correlation for above parameters was proposed, respectively.

Nano-catalyst filter and production method for same

The interfacial shear stress and liquid film thickness are very important to establish the basic model of annular flow in practical applications. In this paper, the liquid film thickness of annular flow in a narrow vertical rectangular channel with a cross-sectional area of 70 mm × 2 mm was measured based on the PCB liquid film sensor. According to the experimental results, the average liquid film thickness, base film thickness and interfacial friction factor were calculated and analyzed. The experimental results showed that the liquid film thickness was affected by the gas phase velocity and the liquid phase velocity, but the effect of the gas phase velocity on the liquid film thickness decreased gradually with increasing gas phase velocity. Moreover, the interfacial friction factor was obvious affected by the liquid phase velocity at low gas phase velocity, but this influence gradually decreased with increasing gas phase velocity. Existing correlations for above parameters were summarized and evaluated with the current data, and there was a large deviation between the predicted results and the experimental results. Therefore, a new correlation for above parameters was proposed, respectively.

Studying Disturbance Wave Velocity and Wall Shear Stress of Vertical Upward Annular Flow in Narrow Rectangular Channel

As two important parameters, the velocity of disturbance wave and the wall shear stress in annular flow are very important to solve the closed equations of the mechanical model for annular flow. In this study, the disturbance wave velocity and wall shear stress of annular flow in a vertical narrow rectangular channel with a cross section of 70 mm × 2 mm were studied. According to the experimental results, it is found that the wave velocity and wall shear stress of disturbance wave increase with increasing gas phase velocity and liquid phase velocity. Also, existing correlations for predicting the velocity of disturbance wave were summarized and evaluated using the current experimental data. A new correlation for wall shear stress based on the disturbance wave velocity has been proposed. Compared with the existing correlation for predicting wall shear stress, this new correlation can well predict the current experimental data and MAPE is only 7.32%.

An evaluation of critical heat flux prediction methods for the upward flow in a vertical narrow rectangular channel

Narrow rectangular channels have widespread application in various domains owing to their significant enhancement in boiling heat transfer. In the present work, the CHF (critical heat flux) prediction method has been comprehensively evaluated and analyzed by the experimental data covering wide operating conditions in narrow rectangular channels. The Bubble Crowding Model can predict the high critical heat flux region. The prediction deviation of ANN (Artificial Neural Network) with two hidden layers and five neurons to predict CHF can reach 20 %, then the influences of thermal-hydraulic parameters on prediction of CHF were obtained based on this constructed ANN. Among the tens of CHF correlations, Tong's, Sudo's, and Mudawar's correlations were selected, and the results indicate that Sudo's correlation can forecast well in 1–4 MPa and Mudawar's correlation had relatively low prediction errors for a wide pressure range. The LUT (look-up table) needs proper correction factors to accurately predict the CHF in narrow rectangular channels.

Experimental investigation and a mechanical model of critical heat flux in a narrow rectangular channel

CHF is the maximal limit of heat flux in boiling heat transfer, therefore, it is a crucial parameter that has a fundamental impact on the security and economy of the nuclear system. An experimental investigation was carried out in the present study to identify the dominant CHF mechanism in a narrow rectangular channel at different gap sizes (2 mm and 3 mm). The visualization experiments were performed at pressures ranging from 1 to 4 MPa, inlet subcooling from 60 to 120 K, and mass flux of 350–2000 kg/(m2·s). A theoretical model of DNB-type CHF has been proposed through visualization phenomena and bubble dynamics analysis. A set of constitutive correlations is developed to close the model. An assessment of the new model has been conducted using the experimental data for the upward flow in a vertical narrow rectangular channel. The predicted results show good accuracy, which is less than 30% relative to the present CHF data and other CHF data under high-pressure conditions.

Development of force balance model for prediction of bubble departure diameter and lift-off diameter in subcooled flow boiling

Visualization experiments are carried out and bubble departure diameter is obtained for subcooled flow boiling in a vertical narrow rectangular channel. Force balance analysis of a growing bubble is performed to determine the criteria for bubble departure and lift-off. A new mechanistic force balance model for predicting bubble departure and lift-off diameter is developed by modifying the bubble growth rate, bubble contact diameter and bubble contact angles. A detailed variation of forces with bubble size is presented, which is used to explain the bubble behaviors intuitively. Compared with Klausner's and Yun's force balance models, the present model shows a good agreement with experimental data by Zhou, Guan, Okawa and Ren. The results also show that the new model can be used to predict bubble diameters in various geometric channels.

Classification and modeling of flooding in vertical narrow rectangular and annular channels according to channel-end geometries

In this study, data classification and modeling of the flooding phenomenon were performed in a narrow channel with a gap thickness of several millimeters. To implement the conventional classification study to a narrow channel case, three key ideas for two-phase flow in a narrow channel were proposed; characteristic length, the flow path for each phase and a concept of unit-cell that can unify both narrow rectangular and annular channels including large outer diameter. Based on the ideas, 330 flooding data points for both narrow rectangular and annular channels including large outer diameter were classified into exit flooding and entrance flooding according to the geometries of the liquid inlet. In order to develop the prediction model for flooding in a narrow channel, the hyperbolicity breaking concept was introduced. The present flooding model was derived based on the hyperbolicity breaking of the two-fluid model and produced the Root Mean Square Errors (RMSE) of 30.02% in terms of the superficial gas velocity against the 330 flooding data points from narrow rectangular and annular channels with relatively small outer diameter. Moreover, the proposed model was applied to a case of narrow-large diameter annuli with the introduction of the number of the unit-cells producing the RMSE of 26.11% against 91 flooding data points. The RMSEs of the existing models for narrow rectangular and narrow large-diameter annuli were 38.25% and 31.10%, respectively. Different from the existing models, the proposed model turned out that it is applicable to various narrow channel-types; a rectangular, an annular and an annulus with a large diameter.

Two-phase flow regime in a vertical narrow rectangular channel

Two-phase flow regime in a vertical narrow rectangular channel

Visualized Experiment of Bubble Behaviors in a Vertical Narrow Rectangular Channel Under Natural Circulation Condition

The characteristics of bubble behavior have been in particular interest for decades due to its significant contribution to understanding the mechanism of heat transfer. In the present work, visualized experiment is conducted to study the bubble characteristics in subcooled flow boiling of a narrow rectangular channel under natural circulation. The experiments were performed at pressures of 0.2 MPa, with inlet subcooling ranging from 20 to 60 K and heat flux ranging from 100 kW/m2 to 300 kW/m2. A high-speed digital camera is used to capture the pictures of bubble behaviors. A sequence of image processing algorithms is used deal with the original bubble images to get relevant bubble parameters. We observe the whole process of a single sliding bubble lifetime and found most of bubbles slide along the heating surface after detaching the nucleation sites. Five typical sliding bubble growth paths are observed in the present experimental conditions. According to the analysis of the experimental data, it can be found that the liquid subcooling and wall superheat are the main factors that affect the bubble size during sliding in narrow rectangular channel under natural circulation condition. Due to the difference of driving force, the sliding velocity of bubble in forced circulation is always larger than that in natural circulation. At the same time, the bubble velocity changes significantly at different heat flux and shooting location.

Flow patterns of vertically upward and downward air-water two-phase flow in a narrow rectangular channel

Considering the importance of two-phase flows in narrow rectangular channels and their use in various applications, the hydrodynamics of a co-current air-water two-phase flow in a vertical narrow rectangular channel has been studied. Experiments have been carried out and flow regime maps for vertical upward and vertical downward flows have been plotted based on measured data sets. Flow regimes were identified and classified based on visual observation and void fraction data. Flow visualization was performed using a high speed camera, while a void fraction analysis was done using the electrical impedance method and a digital image analysis. Four different flow patterns were identified for the vertical upward flow and seven flow patterns were identified for the vertical downward flow. The flow regime map for the vertical upward flow was compared with previous studies and also with the flow regime map obtained for the vertical downward flow in this study.

Two-Phase Flow Patterns and Evaporation Heat Transfer of R134a in a Vertical Narrow Rectangular Channel

This study experimentally investigated the flow pattern, void fraction, and evaporation heat transfer characteristic of R134a upward flow in a vertical narrow rectangular channel having a hydraulic diameter of 0.99[Formula: see text]mm, resembling a plate heat exchanger. Experiments were performed with mass velocities and vapor quality ranging between 30–200[Formula: see text]kgm[Formula: see text]s[Formula: see text] and 0.05–0.9, respectively, at a saturation temperature of 15[Formula: see text]C. Flow patterns were classified into bubble, slug, churn, and annular flows. The void fraction increased with increasing quality, and decreased with decreasing mass velocity in the low-quality region. Further, the influence of flow inlet/outlet positions remarkably appeared when the superficial gas velocity was low. The observed flow patterns and the measured void fraction were compared to that in previous studies. The effects of mass velocity and heat flux on the evaporation heat transfer coefficient were small, and the heat transfer through the thin liquid film was dominant.

Visual observations of flow patterns in downward air-water two-phase flows in a vertical narrow rectangular channel

Flow patterns in downward air–water two-phase flows in a narrow rectangular channel were visually observed. A test section was constructed using transparent acrylic plate with a test channel 2.35 mm thick, 66.6 mm wide, and 780 mm long. The experiments determined that at high liquid velocity the flow patterns for downward flow are similar to those observed in upward flow. However, at low liquid velocity some significant differences were observed between downward and upward flows. The new flow patterns, falling film flow, large-bubbly flow, and intermediate flow, were identified in the low liquid velocity region, where buoyancy force is influential. A flow regime map for downward flow in a narrow rectangular channel was constructed using the experimental data and analyses.

Effect of transverse power distribution on the ONB location in the subcooled boiling flow

This study investigates the effect of transverse power distribution on the ONB (Onset of Nucleate Boiling) incipient. For this purpose, a subcooled boiling model with uniform and non-uniform heat flux distribution is simulated in a narrow vertical rectangular channel heated from both sides by applying a wide range of thermal power (8–16kW). The simulations are performed using the CFX and TMAP codes. The CFX code incorporates both a two-fluid model and RPI wall boiling model to investigate coolant and wall temperature distributions along the heated channel. The TMAP code implements two different sets of heat transfer correlations to evaluate the wall temperature. The results obtained from the TMAP analyses show that the wall temperatures predicted by the Jo et al. heat transfer correlation are higher than the ones predicted by the Dittus and Boelter heat transfer correlation. The wall temperatures predicted by the CFX analyses lie between the predicted wall temperatures obtained by the TMAP analyses. Based on the superheated temperature on the heated surface, the ONB incipient is determined. The axial locations of the ONB incipient are predicted differently by the CFX and TMAP analyses. For uniform heating, the ONB incipient predicted by the CFX analysis occurs between the predictions made by the TMAP analyses. For non-uniform heating, the ONB incipient by the CFX analysis occurs at a higher power than the power required by the TMAP analyses.

Criteria for Onset of Flow Instability in heated vertical narrow rectangular channels at low pressure: An assessment study

In this paper, an assessment of the criteria for the prediction of the flow excursion instabilities in vertical narrow rectangular channels is presented. The experimental database consists of 166 flow excursion points at low pressure with upward flow, and with uniform or non-uniform heat flux profiles. The test sections have gap sizes between 1.27 and 3.6mm, hydraulic diameters between 2.37 and 6.58mm, and length to heated diameter ratio between 70.9 and 196.8. A wide range of parameters is covered: the mass flux is between 740 and 20,325kg/m2s; the outlet pressure is between 0.12 and 1.73MPa; the heat flux is between 0.4 and 14.9MW/m2; the Peclet number is between 15,889 and 358,460; the outlet sub-cooling is between 4.8 and 35.1°C. None of the tests reaches saturation at the exit of the test section.Several criteria for identifying the Onset of Flow Instability (OFI), were tested. Such criteria can rely on correlations for the Onset of Nucleate Boiling (ONB), the Net Vapor Generation (NVG), the onset of Fully Developed Boiling (FDB), or can relate global parameters of the system. All these models have good performances on average, with both uniform and non-uniform axial heat fluxes.The ONB-based relationships are largely conservative as expected since the ONB always precedes the OFI. The NVG criteria can provide relatively good results, but a crucial issue is related to the value of Peclet number at which the transition between the thermally and the hydro-dynamically driven bubble detachment takes place in narrow rectangular channels. In view of this, the standard Saha–Zuber correlation cannot predict OFI for Peclet numbers lower than 70,000; while the Saha–Zuber KIT correlation, whose transition Peclet number is smaller, identifies the OFI for all the experiments. The approach that makes use of a FDB correlation can capture the OFI in most of the cases, although its performance also depends on the type of correlation that is applied for the single-phase heat transfer. The Flow Instability Ratios (FIRs) like the ones developed by Whittle–Forgan or Stelling et al., are of particular interest because they only require global system parameters, and because they are shown to be a valid option for determining the flow excursion in the experiments included in this study. For instance, the Stelling FIR with the Saha–Zuber KIT correlation estimates OFI in all the tests. Finally, best-fitting procedures of the available data were also introduced in order to optimize such FIRs.