Liquid transport during gas flow transients applied to liquid loading in long vertical pipes

An experimental study was carried out to investigate the liquid distribution and transport in vertical gas–liquid flows in a 42 m long, 0.048 m ID tube system. Liquid loading in gas wells is generally defined as the inability of the produced gas to lift the co-produced liquid up the tubing, resulting in liquid accumulation in the wellbore. The characterization of the liquid loading phenomenon is often based on field monitoring, with limited measurements of pressure and liquid holdup profiles, and usually without visual observations of the phenomenon. The experimental observations obtained in this work show the liquid re-distribution during changes in gas flow rate. These observations revealed that, after a change in gas rate, the liquid film starts to flow downwards in the pipe, and subsequently, the liquid begins to steadily flow upward at a constant velocity. The experimental data were compared with a numerical model, showing an agreement of ±25%. A pseudo-steady state approach was used to model the vertical gas–liquid flows in this long pipe to describe liquid loading in vertical wellbores. The liquid droplets entrained in the gas core were observed and estimated to be flowing upwards in order to obtain a good agreement between experimental observations and model results, even though the gas velocities were lower than Turner critical velocities. The widely accepted droplet model of Turner says that below the critical velocity, the liquid droplets should be flowing downward and not upwards

Axial development of annular, churn and slug flows in a long vertical tube

Experimental data are presented on the axial development of gas–liquid flows in a 42-m long, 0.048-m ID tube system. Different transitional models for slug, churn and annular flow regimes were compared with visual observations and showed reasonable agreement for locations L/D = 560 and 820. The behavior of liquid holdup and frequency of flow structures (large waves, disturbance waves) were also analyzed for three different axial positions (L/D = 102, 521 and 815), over a range of pressure between 1.4 and 5.2 bar, liquid mass fluxes ranging from 17 to 319 kg/m2-s and dimensionless gas velocities from 0.05 to 1.6. The liquid holdup showed significant axial variation at all monitoring locations for higher liquid mass fluxes, while it appeared that a kind of developed flow was reached at L/D = 521 for lower liquid mass fluxes. The flow structure frequency did not exhibit significant axial variation for dimensionless gas velocities (Wallis parameter) between 0.2 and 1.6

Experimental investigation on the prediction of liquid loading initiation in gas wells using a long vertical tube

Liquid loading in gas wells is generally defined as the inability of the produced gas to lift the co-produced liquid up the tubing, resulting in liquid accumulation in the wellbore. This liquid accumulation is related to decrease in gas production and it can even cease production. The characterization of this phenomenon is often based on field monitoring, with limited measurements of pressure and liquid holdup profiles, and usually without visual observations of the phenomenon. This paper reveals there is still a significant discrepancy between the available models and the experimental data available in the literature. Besides reviewing the present state of research into this area, the current understanding about the mechanisms of liquid loading initiation is discussed in this paper and directions on how to use this understanding to model and provide solutions to liquid loading problems is also presented. An experimental investigation on the initiation of liquid loading in a gas well has been carried out using a vertical 42 m long, 0.0489 m (2 in) ID tube system. Vertical gas–liquid flows were analyzed with the aim of understanding how the liquid accumulates in a long vertical tube while decreasing the gas flow rate. This experimental study includes visual observations of the liquid transport and two-phase flow regimes, and measurements of pressure, temperature and liquid holdup along the vertical tube. The experimental observation of the liquid build-up was correlated to prediction criteria models available in the literature for liquid loading initiation

A study of the air-side heat transfer and pressure drop characteristics of tube-fin [`]no-frost' evaporators

A study is presented on the influence of the air flow rate and surface geometry on the thermal-hydraulic performance of commercial tube-fin [`]no-frost' evaporators. A specially constructed wind-tunnel calorimeter was used in the experiments from which data on the overall thermal conductance, pressure drop, Colburn j-factor and Darcy friction factor, f, were extracted. Eight different evaporator samples with distinct geometric characteristics, such as number of tube rows, number of fins and fin pitch were tested. Semi-empirical correlations for j and f are proposed in terms of the air-side Reynolds number and the finning factor. A discussion is presented on the performance of the evaporators with respect to specific criteria such as the pumping power as a function of heat transfer capacity and the volume of material in each evaporator.Tube-fin evaporator Experimental analysis Correlation Frost-free refrigerator

Modeling transient churn-annular flows in a long vertical tube

This work investigates the transient behavior of high gas fraction gas-liquid flows in vertical pipes (annular and churn flows). Hyperbolic balance equations for mass, momentum and entropy are written for the gas and liquid, which is split between a continuous film and droplets entrained in the gas core. Closure relationships to calculate the wall and interfacial friction and the rates of droplet entrainment and deposition were obtained from the literature. A finite-difference solution algorithm based on a coefficient matrix splitting method was implemented to deal with sharp variations in the spatial and temporal domains, such as pressure and phase holdup waves. The model results were compared with steady-state experimental data from eight different sources, totaling more than 1500 data points for pressure gradient, liquid film flow rate and void/core fraction. The absolute average deviation between the model and the data was 17% for the pressure gradient and 5.8% for the void fraction. A comparison of the model results with fully transient air-water data generated in a 49-mm ID, 42-m long vertical pipe is also presented. The experimental results consist of two outlet pressure-induced and two inlet mass flow rate-induced transient tests. Two main transient parameters are compared, namely the local void fraction and the pressure difference between selected points along the test section and the outlet (taken as a reference). The comparisons between the experiments and the numerical model indicate that the model was capable of describing the transient annular to churn flow transition with absolute average deviations of 14.5% and 7.9% for the pressure difference and void fraction, respectively