Numerical investigation on the performance of coalescence and break-up kernels in subcooled boiling flows in vertical channels

In order to accurately predict the thermal hydraulic of two-phase gas-liquid flows with heat and mass transfer, special numerical considerations are required to capture the underlying physics: characteristics of the heat transfer and bubble dynamics taking place near the heated wall and the evolution of the bubble size distribution caused by the coalescence, break-up, and condensation processes in the bulk subcooled liquid. The evolution of the bubble size distribution is largely driven by the bubble coalescence and break-up mechanisms. In this paper, a numerical assessment on the performance of six different bubble coalescence and break-up kernels is carried out to investigate the bubble size distribution and its impact on local hydrodynamics. The resultant bubble size distributions are compared to achieve a better insight of the prediction mechanisms. Also, the void fraction, bubble Sauter mean diameter, and interfacial area concentration profiles are compared against the experimental data to ensure the validity of the models applied

Numerical investigation on bubble size distribution around an underwater vehicle

The interaction of bubbles with flow boundaries has been of high interest for marine engineering; for example, when a propeller is cavitating or air is entrained in the wake of a maneuvering ship, the strong interaction with the boundary layer will lead to the formation of a bubbly wake. To be able to develop the best mitigation strategy, a deep understanding of the associated physics is required. Only a few articles published in open literature address this two-phase fluid system. In the present study, a 3D numerical simulation has been performed to model a bubbly two-phase flow around the DARPA SUBOFF, in which exhaust gas is discharged into the flow around the object to provide a platform for investigating the distribution of the bubbles around a curved body. The two-phase flow is modelled using the Eulerian-Eulerian approach coupled with the MUSIG model. The bubble distribution is characterized based on different gas discharge configurations. It has been found that the boundary layer flow has a strong effect on bubble formation process, particularly encourages the bubble fragmentation. As a result, many small bubbles will be trapped at the aft of the vehicle

Modelling subcooled flow boiling in vertical channels at low pressures - Part 1: Assessment of empirical correlations

Modeling subcooled flow boiling in vertical channels at low pressures requires not only the consideration of the dynamic behaviors of two-phase flow and bubbles undergoing coalescence, breakup and condensation in the bulk subcooled liquid but also the characterization of the single-phase and local boiling heat transfer phenomena in the near-wall region. The focus of this paper is the assessment of the heat flux partitioning model in handling the latter physics of subcooled flow boiling. In order to achieve closure to the model, the current prevailing approach has always been the utilization of empirical correlations particularly for the active nucleation site density, bubble departure diameter and bubble departure frequency. A comprehensive survey of existing empirical correlations is presented in the first part of this paper to assess the performance of these empirical models. Selected combinations of empirical correlations are compared and validated against axial and local radial experiments encompassing a wide range of different mass and wall heat fluxes and inlet subcooling temperatures for subcooled flow boiling at low pressures. Based on the comparisons made against the axial and local distributions of void fraction and bubble Sauter diameter, not one single combination of empirical correlations has shown the propensity of providing satisfactory predictions covering the entire axial and local conditions. For the modeling of subcooled flow boiling at low pressures to become an effective predictive tool, it must therefore be complemented with further consideration of first principal models of the underlying physical phenomena

Modelling subcooled flow boiling in vertical channels at low pressures - Part 2: Evaluation of mechanistic approach

In this paper, the improved heat flux partitioning model based on first principal models of the underlying physics in determining the active nucleation site density, bubble size and bubble frequency is evaluated for subcooled flow boiling in vertical heated channels at low pressures. Salient features of this model include the fractal approach in determining the active nucleation site density, the force balance model in determining bubble diameters at sliding and lift-off as well as bubble frequency through the consideration of growth and waiting times and the additional heat flux at the heated wall due to surface quenching of sliding bubbles. Model predictions, covering a wide range of different mass and heat fluxes and inlet subcooling temperatures, are compared against local and axial measurements. In comparison to the measured data and selected combinations of empirical correlations such as described in Part 1, the current model predictions clearly demonstrate the effect of the subcooling effect on the activation of nucleation sites at the heated wall. The selected combinations of empirical correlations consistently under-estimate the wall superheat temperature while the current model yields predictions that agree very well with experimentally measured temperatures. Bubble sliding along the heat wall and its influence on heat partitioning and surface quenching heat flux has been ascertained to play an important role during the subcooled flow boiling process

Efficiency of a two-phase nozzle for geothermal power generation

In this paper, the performance of a two-phase nozzle, as an expander to generate power is studied. Mostly high pressure and temperature energy sources are investigated in the literature, whereas the possibility of utilizing low temperature energy remains sparsely covered. This paper aims to bridge this knowledge gap. Experiments involve with passing water having temperatures lower than 100 °C through a convergent-divergent nozzle to a low pressure flash tank. The thermal energy of water is converted to kinetic energy in the nozzle. Then by analysing the measured data, the efficiency of the process is evaluated. The pressure and temperature profiles along the nozzle are obtained and compared with the saturated condition. The experimental data provide essential information towards understanding the complex flashing process in the nozzle. The results complement the available data on two phase nozzles for medium to high temperature applications

Experimental performance of a rotating two-phase reaction turbine

This paper presents the experimental performance of a curved nozzle two-phase reaction turbine design for trilateral flash cycle heat engine. Results from two tests have been presented, for first test the average feed water temperature was maintained around 97 C under local atmospheric pressure, while for the second test the average feed water temperature was maintained around 115 C under 200 kPa absolute pressure. For both the tests the initial condenser pressure was maintained around 6 kPa absolute. The maximum power output of the turbine is estimated to be around 1330 W with an isentropic efficiency of around 25%

The efficiency of a two-phase nozzle as a motion force for power generation from low-temperature resources

Two-phase nozzles could be used as energy conversion devices in geothermal total flow systems or binary fluid systems followed by a trilateral cycle for power generation. In this paper, the efficiency of such nozzles is investigated. Also, a profound research has been done on similar area in the past where mostly high pressure and high temperature energy resources were considered; so, the possibility of utilizing low-temperature energy resources remains limited in the literature. In order to bridge the knowledge gap, the feasibility of utilizing low-temperature resources for power generation is studied in this paper. In this regards, experiments are carried out with the following conditions: a convergent-divergent nozzle is supplied with water at atmospheric pressure with various temperatures at/below 100ºC. This nozzle is connected to a tank that is evacuated by a vacuum pump. The driving force for water to flow through the nozzle is the pressure difference between atmosphere and vacuum pressure in the flash tank. As water is passed through the nozzle, the thermal energy is converted to kinetic energy as a motive force for power generation. The impulse force caused by the jet exiting the nozzle is measured and compared against the ideal case (i.e. isentropic expansion assumption) to calculate the thrust coefficient of the nozzle and evaluate the efficiency of the process. Also, the pressure and temperature profiles along the nozzle are obtained and compared against saturation pressure corresponding to measured temperatures. The results encourage the utilization of low-temperature geothermal energy resources for power generation

Investigation of the Influence of Elevated Pressure on Subcooled Boiling Flow - Model Evaluation Toward Generic Approach

Modeling subcooled boiling flows in vertical channels has relied heavily on the utilization of empirical correlations for the active nucleation site density, bubble departure diameter, and bubble departure frequency. Following the development and application of mechanistic modeling at low pressures, the capability of the model to resolve flow conditions at elevated pressure up to 10 bar is thoroughly assessed and compared with selected empirical models. Predictions of the mechanistic and selected empirical models are validated against two experimental data at low to elevated pressures. The results demonstrate that the mechanistic model is capable of predicting the heat and mass transfer processes. In spite of some drawbacks of the currently adopted force balance model, the results still point to the great potential of the mechanistic model to predict a wide range of flow conditions in subcooled boiling flows

Numerical investigation on the bubble size distribution around NACA0015 hydrofoil

Two-phase bubbly flows occur widely in nature and are extensively applied in industry. The aeration processes underwater is one type of two-phase bubbly flow that directly impacts on the downstream water quality by reducing the oxygen content in the water. The most important influencing factors for optimization design of Auto-venting turbines (AVTs), for solving the low level of dissolved oxygen (DO) in the discharged downstream water, are the quantity of entrained air, the bubble size distribution resulting from coalescence and breakage processes, and the rate of oxygen transfer from the bubbles. In order to better understand the influencing flow conditions on the bubble size distribution, in this paper a numerical investigation for flow around NACA0015 hydrofoil is carried out. The numerical simulations require the consideration of the dynamic behaviors of two-phase flow and bubbles undergoing coalescence and breakup. For this purpose, the ensemble-averaged mass and momentum transport equations for continuous and dispersed phases are modeled within the two-fluid modeling framework. These equations are coupled with population balance equations (PBE) to aptly account for the coalescence and break-up of the bubbles. The resultant bubble size, normalised velocity and void fraction distributions for different flow conditions including angle of attack (AOA), air-entrainment coefficient, and Reynolds number are presented and discussed. The results show that varying AOA has the most significant impact on the distribution of the bubbles in the wake