Study on the similarity of wet gas pressure drop in long-throat Venturi

This study investigates the effect of pipe diameter on pressure drop with the same diameter ratio, similar pressure-sampling position and throat length of long-throat Venturi. Considering the factors including the void fraction, the friction between the two phases and the entrainment in the gas core, the one-dimensional momentum equation for gas has been solved in the axial direction of long-throat Venturi. A novel void fraction model is established, by considering the effects of dryness and gas-liquid density ratio, then predicting the distribution of wet gas static pressure between the two pressure tapings of the long-throat Venturi. The comparison between the values predicted by the model and those measured experimentally reveals that all the relative deviations of the predicted points by the modified model were within ±15%. In the same entrance conditions, the effect of pipe diameter on pressure drop in long-throat Venturi is similar.     

Prediction of pressure drop in Venturi based on drift-flux model and boundary layer theory

Venturi, as the primary flow measurement sensor, is widely used in various industrial fields of oil and natural gas. Pressure drop of the Venturi is a crucial factor in the process design of exploitation and transportation of natural gas. Based on the drift-flux model and boundary layer theory, a pressure drop prediction model is established. Except for divergent section, a uniform void fraction model is established basing on drift-flux model. The thickness of boundary layer grows rapidly due to the existence of adverse pressure gradient in the divergent section, which results in an increase of the irrecoverable pressure drop. Considering the influence of slip between gas and liquid, weight coefficient is used to adjust the proportion of displacement thickness in the cross section of the Venturi. Compared to experiment, the theoretical model is applied to stratified wavy flow and annular mist flow. For different diameter, the relative deviations of experiment points are within ±15%.     

Air–water two-phase flow measurement using a Venturi meter and an electrical resistance tomography sensor

A method for air–water two-phase flow measurement is proposed using a Venturi meter combined with an Electrical Resistance Tomography (ERT) sensor. Firstly, the real-time flow pattern of the two-phase flow is identified using the ERT sensor. Secondly, the void fraction of the two-phase flow is calculated from the conductance values through a void fraction measurement model, developed using the LS-SVM regression method. Thirdly, the mass quality is determined from the void fraction through void fraction-quality correlation. And finally, the mass flowrate of the two-phase flow is calculated from the mass quality and the differential pressure across the Venturi meter. Experimental results demonstrate that the proposed method is effective for the measurement of the mass flowrate of air–water flow. The proposed method introduces the flow pattern information in the measurement process, which minimizes the influence of flow pattern on the conventional differential pressure based methods. In addition, the mass quality is calculated from the void fraction, so the difficulty to obtain the mass quality in conventional methods is also overcome. Meanwhile, the new method is capable for providing concurrent measurements of multiple parameters of the two-phase flow including void fraction, mass quality and mass flowrate as well as an indication of the flow pattern.     

Numerical simulation of vapor volume fraction in a vertical channel under low-pressure conditions

This numerical study involved investigating void behavior under low-pressure subcooled flow boiling by using an Eulerian approach (two-fluid model). In the simulation, a vertical pipe with a length of 0.15 m and diameter of 0.01229 m was considered. Different levels of uniform wall heat flux, mass flux, and inlet subcooling temperature were applied although a constant pressure of 1.65 bar was used for all the simulations. A sensitivity study of the empirical coefficients used to access the predictive capacity of the existing mass transfer models was conducted. Thus, the k-epsilon model was used for the turbulence of the fluid. The axial vapor volume fraction profile, liquid temperature is compared at the operating pressure. Furthermore, the most sensitive flow characteristics of the channel were identified. The results indicated that the predictions of numerical phase evolution relative to the experimental observations were in good qualitative agreement with those obtained in extant studies. Additionally, the changes in drag coefficients were helpful in precisely predicting the void fraction. A commercial CFD solver was used for the implementation of the model.     

Gas–oil two-phase flow measurement using an electrical capacitance tomography system and a Venturi meter

A new flowrate measurement method for gas–oil two-phase flow using an electrical capacitance tomography technique and a Venturi meter is reported in this paper. A hybrid image reconstruction algorithm combining the Tikhonov regularization and the Algebraic Reconstruction algorithm is used to obtain images. The cross-sectional void fraction is determined from the images. A Venturi meter is used as a velocity measurement device whilst five measurement models are introduced to predict the total mass flowrate of gas–oil two-phase flow. Experimental results obtained show that the performance of the proposed flowrate measurement system is effective.     

Pressure drop of wet gas flow with ultra-low liquid loading through DP meters

The most common method to predict the gas and liquid flow rates in a wet gas flow simultaneously is to use dual pressure drops (dual-DPs) from two or even one single DP meter. In this paper, the metering mechanism of applying dual-DPs were overviewed. To fully understand the response of DP meters to wet gas flows, the pressure drops of wet gas flow with ultra-low liquid loading through three typical DP meters were experimentally investigated, including an orifice plate meter, a cone meter and a Venturi meter. The equivalent diameter ratio is 0.45. The experimental fluids are air and tap water. The pressure is in the range of 0.1–0.3 MPa and the Lockhart-Martinelli parameter (X_LM) is less than approximately 0.02. The results show that the upstream-throat pressure drop, the downstream-throat pressure drop and the permanent pressure loss of individual DP meters have unique response to liquid loading. The upstream-throat pressure drop of the orifice plate meter decreases at first and then increases as the liquid loading increases, while that of the cone meter and the Venturi meter increase monotonically. The non-monotonicity of the pressure drop for the orifice plate meter can be attributed to the flow modulation of trace liquid. The downstream-throat pressure drops of all the three test sections decrease at first and then increase. The reason is that the liquid presence in a gas flow increases the downstream friction and vortex dissipation. The permanent pressure loss of the orifice plate meter also shows non-monotonicity. To avoid non-monotonicity, the pressure loss ratio is introduced, which is defined as the ratio of the permanent pressure loss to the upstream-throat pressure drop. Results show that the pressure loss ratio of the Venturi meter has the highest sensitivity to the liquid loading.     

Wet gas pressure drop across orifice plate in horizontal pipes in the region of flow pattern transition

As a basis for measuring the mass flow rate of wet gas using differential pressure meters, predicting the pressure drop of a wet gas flowing through orifice plates is important; however, this has not yet been solved satisfactorily, although many studies have reported on that subject. In this study, the pressure drop of wet gas across sharp-edged orifice plates was experimentally investigated in the region of flow pattern transition using air and water as the two phases, and the prediction performance of the available pressure drop models was compared based on the experimental data. The results show that the homogenous flow models overestimate the pressure drop, whereas those models based on the separated flow model often present underestimations. The models reported for wet gas are also incapable of predicting the pressure drop in this region with acceptable accuracy. Through an analysis of the prediction deviations, it is found that the Froude number of the liquid phase has a significant influence on the pressure drop of the wet gas, besides the Froude number of the gas phase. Then, three new correlations that are based on the homogeneous flow, Chisholm model, and Murdock model, respectively, were proposed based on the experimental result.     

Coupled model of dual differential pressure (DDP) for two-phase flow measurement based on phase-isolation method

Phase-isolation is a novel ever-increasing multiphase separation technology, which can facilitate the multiphase fluid flowing concurrently with a substantially clear interface between two phases, and the phenomenon is promisingly employed for the separation and measurement of multiphase flows. Phase-isolation can be implemented by different kinds of lateral forces, of which the centrifugal force induced by the swirlers is the most convenient method. The radial pressure drop between pipe wall and pipe center, and the axial pressure drop along the pipe wall occurs at the downstream of the swirler. In the paper, the coupling model of dual differential pressure (DDP) including the radial-axial differential pressure and radial-radial differential pressure was built employing centrifugal phase-isolation for oil-water two-phase flow, and the theoretical measurement models were validated by our experimental data. At certain cross sections downstream of the swirler, the deviations between theoretical and experimental result of the volumetric oil fraction λ_o and mass flowrate Q_m were below ±7.16% and ±1.14% respectively when the radial-axial differential pressure was adopted, while the deviations between theoretical and experimental result of λ_o and Q_m were below ±6.91% and ±1.13% respectively using the radial-radial differential pressure. The acceptable deviation indicates that the DDP model can be the reference for the analysis and application of two-phase flow in the academic research and practical engineering.     

Determination of void fraction in wet-gas vertical flows via differential pressure measurement

The measurement of void fraction in multiphase flow is important for a wide range of industrial processes. Existing methods for void fraction measurement require intrusive, expensive and potentially hazardous equipments which constrict the flow, adding both capital and operational costs. Two phase flow experiments were carried out at the National Engineering Laboratory (NEL) to measure void fraction via pressure drop in a vertical pipe. Additional experiments are carried out at Spirax Sarco Inc. to validate the efficacy of the method on steam/water flow mixtures at high temperature and pressure, in gas mass fraction range between 0.17 and 0.95 and void fraction range between 0.75 and 1.0. The void fraction calculated by the presented differential pressure (dP) method is confirmed via established correlations. The work demonstrates the efficacy of a low cost, non-intrusive method to determine void fraction in two phase flow over a wide range of flow conditions.     

微通道中液氮的流动沸腾——两相流动压降分析

对液氮在直径为0.531 mm,加热长度为250mm的圆管中的流动沸腾压降和传热特性进行研究.作为第一部分,主要对微通道中液氮的两相流动压降进行试验研究与分析.结果表明:在核态沸腾起始时,质量流量迅速降低,而压降突然增大,并伴随着明显的温度滞后,幅度约为4.0～5.0 K.由于压降很大,在微通道内液氮的两相流动中会出现闪蒸,从而对质量干度产生重要影响.最后,利用均相模型和三个两相流动模型(L-M模型,Chisholm B系数模型和Friedel模型)对微通道沿程压降进行分析和比较.不同于常规通道的是,均相模型可以很好地预测压降试验结果,而三个两相流动模型的预测偏差较大,这是由于在微小通道中的高速流动情况下,汽相和液相混合比较均匀;同时液氮的液汽密度比很小,这也有利于均相模型的预测.