An evaluation of the performance of phenomenological models for predicting pressure gradient during gas-liquid flow in horizontal pipelines

Direct steam generation collectors are considered with the aim to improve the performance of a parabolic trough collector leading to a reduction of operating costs of solar electric generation systems. In this study a hydrodynamic steady state model is developed and linked with a thermal model to optimize the performance of once-through direct steam generation solar collectors. The hydrodynamic model includes flow pattern classification and a pressure drop model. Flow pattern maps for typical DSG collectors with horizontal and inclined absorber tubes are generated to investigate the variation of flow conditions with radiation level, tube diameter, tube length and flow rate. Two-phase flow frictional pressure drop correlations for the range of operating conditions in a DSG collector are selected from the wide range of published correlations by comparison with experimental data for typical steam-water flow conditions in a DSG collector. Pressure drop is calculated for different operating conditions for both horizontal and inclined solar absorber tubes. Alternative operational strategies are evaluated to achieve optimum performance of a direct steam generation collector at different radiation levels.

Hydrodynamic Analysis of Direct Steam Generation Solar Collectors

Direct steam generation (DSG) in parabolic-trough collectors (PTC) is one of the most attractive technologies in concentrated solar power plants. Its appeal stems from its ability to reduce the operational and maintenance costs compared with other heat transfer fluids. Modelling and simulation (M&S) tools, together with the development of experimental real-scale set-ups have played a key role in the advancement of this solar technology. The aim of this review is to summarize and analyse the thermohydraulic, thermal and optical models implemented in M&S tools for DSG in PTC in order to identify the contribution that these models could provide towards the improvement of the technology in the future. Thermohydraulic models have been, in most cases, developed under the three-equation homogeneous equilibrium model (HEM) approach, successfully for recirculation mode. The more complete six-equation two-fluid model (TFM) approach, has also been properly applied, to a lesser extent, to modelling the once-through solar field operation mode, considering water/steam two-phase flow patterns. Although these advancements have contributed to the design and operation of the first commercial solar steam power plant with PTC for electricity generation, there are however some technological gaps still to be overcome to consolidate the technology. In recirculation solar field operation mode, the use of HEM has shown to be adequate to model the DSG process in PTC integrated with thermal energy storage systems and into solar hybrid power plants. For once-through operation mode, the distributed-parameter thermohydraulic models, especially under TFM approach, involving a detailed flow pattern map, have demonstrated to be suitable tools for solving the uncertainties related to the two-phase flow, especially at the endpoint of the evaporation section.

Modelling and simulation tools for direct steam generation in parabolic-trough solar collectors: A review

A model for simulating gas bubble entrainment in two-phase horizontal slug flow

Two-phase flow of gases and liquids or vapors and liquids in pipes, channels, equipment, etc. is frequently encountered in industry and has been studied intensively for many years. The reliable prediction of pressure drop in two-phase flow is thereby an important aim. Because of the complexity of these types of flow, empirical or semiempirical relationships are only of limited reliability and pressure drops predicted using leading methods may differ by up to 100%. In order to improve prediction methods, this work presents an experimental and analytical investigation of two-phase pressure drops during evaporation in horizontal tubes. The goal of the experimental part was to obtain accurate two-phase pressure drop values over a wide range of experimental conditions. The existing LTCM intube refrigerant test loop has been modified and adapted to the new test conditions and measurement methods. Two new test sections have been also implemented into the modified test rig. The new test section consists of two zones: diabatic and adiabatic. This configuration allows tests to be run that obtain experimental two-phase pressure drop values under diabatic and adiabatic conditions simultaneously. The experimental campaign acquired 2543 experimental two-phase pressure drop values. Based on a comprehensive state-of-the-art review and comparison with two-phase frictional pressure drop prediction methods, it is proven that none of these methods were able to accurately, reliably predict the present experimental values. In the second part of this work, an analytical study was undertaken in order to develop a new two-phase frictional prediction method. It has been shown in the literature that the so called "phenomenological approach" tends to provide more accurate and realistic predictions as the interfacial structure between the phases is taken into account. Based on that, a phenomenological flow pattern approach was chosen in the present study. The recent Wojtan-Ursenbacher-Thome [155] map was chosen to provide the corresponding interfacial structure. The new model treats each flow regime (i.e. interfacial structure) separately and then ensures a smooth transition in between, being in agreement with the experimental observations. Another important feature of the proposed model is that it matches the correct limits at x = 0 (single-phase liquid flow) and x = 1 (single-phase gas flow). Based on a statistically comparison, it is concluded that the new two-phase frictional pressure drop model based on flow pattern map successfully predicts the new experimental data. The present work completes the fourth basic step in LTCM's flow pattern based work on two-phase flow and heat transfer inside horizontal round tubes: (i) generalized flow pattern map, (ii) flow boiling heat transfer model, (iii) convective condensation model and (iv) two-phase frictional pressure drop model.

Experimental and analytical study of two-phase pressure drops during evaporation in horizontal tubes

Friction pressure drop for gas-liquid flow in horizontal pipes can be estimated within an accuracy of about + or - 30% via a general correlation, which applies to all flow patterns, except dispersed flow.

Z. Olujic

Papers

Predicting two-phase-flow friction loss in horizontal pipes