Bubble shape and breakage events in a vertical pipe at the boiler flow line

The theoretical and experimental aspects concerning the typical bubble shape at the flow line of a standard domestic central heating system are investigated. This is done in support of the on-going research on two-phase flows in domestic central heating systems. Bubble nucleation and detachment at the primary heat exchanger wall of a domestic central heating boiler results in a bubbly two-phase flow in the system pipe work. Bubbly flow results in undesired cold spots at higher points in the system, consequently diminishing system performance. An experimental analysis was done on the bubble shape at the exit of the boiler through the application of photographic techniques. The results are presented in terms of the measured bubble aspect ratios at some principal system operating conditions. The dimensionless Eotvos and bubble Reynolds number were calculated and tabulated with the measured mean diameters. The data was subsequently correlated to the bubble shape regime diagram. Results suggest that most bubbles are quasi-spherical in shape with a noticeable elongation at lower bulk fluid Reynolds numbers

Two-phase distribution in the vertical flow line of a domestic wet central heating system

The theoretical and experimental aspects of bubble distribution in bubbly two-phase flow are reviewed in the context of the micro bubbles present in a domestic gas fired wet central heating system. The latter systems are mostly operated through the circulation of heated standard tap water through a closed loop circuit which often results in water supersaturated with dissolved air. This leads to micro bubble nucleation at the primary heat exchanger wall, followed by detachment along the flow. Consequently, a bubbly two-phase flow characterises the flow line of such systems. The two-phase distribution across the vertical and horizontal pipes was measured through a consideration of the volumetric void fraction, quantified through photographic techniques. The bubble distribution in the vertical pipe in down flow conditions was measured to be quasi homogenous across the pipe section with a negligible reduction in the void fraction at close proximity to the pipe wall. Such a reduction was more evident at lower bulk fluid velocities

Power Generation with Renewable Energy and Advanced Supercritical CO2 Thermodynamic Power Cycles: A Review

Supercritical CO2 (S-CO2) thermodynamic power cycles have been considerably investigated in the applications of fossil fuel and nuclear power generation systems, considering their superior characteristics such as compactness, sustainability, cost-effectiveness, environmentally friendly working fluid, and high thermal efficiency. They can be potentially integrated and applied with various renewable energy systems for low-carbon power generation such that extensive studies in these areas have also been conducted substantially. However, there is a shortage of reviews that specifically concentrate on the integrations of S-CO2 with renewable energy encompassing biomass, solar, geothermal, and waste heat. It is thus necessary to provide an update and overview of the development of S-CO2 renewable energy systems and identify technology and integration opportunities for different types of renewable resources. Correspondingly, this paper not only summarizes the advantages of CO2 working fluid, design layouts of S-CO2 cycles, and classifications of renewable energies to be integrated but also reviews the recent research activities and studies carried out worldwide on advanced S-CO2 power cycles with renewable energy. Moreover, the performance and development of various systems are well grouped and discussed

Two-phase distribution in the vertical flow line of a domestic wet central heating system

The theoretical and experimental aspects of bubble distribution in bubbly two-phase flow are reviewed in the context of the micro bubbles present in a domestic gas fired wet central heating system. The latter systems are mostly operated through the circulation of heated standard tap water through a closed loop circuit which often results in water supersaturated with dissolved air. This leads to micro bubble nucleation at the primary heat exchanger wall, followed by detachment along the flow. Consequently, a bubbly two-phase flow characterises the flow line of such systems. The two-phase distribution across the vertical and horizontal pipes was measured through a consideration of the volumetric void fraction, quantified through photographic techniques. The bubble distribution in the vertical pipe in down flow conditions was measured to be quasi-homogenous across the pipe section with a negligible reduction in the void fraction at close proximity to the pipe wall. Such a reduction was more evident at lower bulk fluid velocities

CFD Modelling of Finned-tube CO2 Gas Cooler for Refrigeration Systems

As a main component in a refrigeration system, finned-tube CO2 gas cooler plays an important role to the system performance and thus needs to be thoroughly investigated. To achieve this, some effective parameters including the CO2 and air fluid velocity fields, temperature profiles and heat transfer characteristics at different operating conditions are predicted and analysed by means of Computational Fluid Dynamics (CFD) modelling and simulation. It is noted that CFD modelling can accurately obtain the local heat transfer coefficients of both air and refrigerant sides, which are difficult to be predicted by conventional empirical correlations. This paper investigates the effect of varied operational parameters on local heat transfer coefficients and temperature profiles of the working fluids in a finned-tube CO2 gas cooler by means of CFD modelling. As one of the simulation results, it is found that the approach temperature decreases with increased air inlet velocity. The model has been compared and validated with experimental measurements and literature correlations. The research methods and outcomes can be used for further investigation and optimization in this area

Performance Analysis of Finned-Tube Heat Exchanger Charged with Phase Change Material for Space Cooling

The performance of a latent heat storage unit comprised of phase change material (PCM) enclosed in a finned-tube heat exchanger was evaluated experimentally and theoretically to determine its viability to condition a space during summer. The internal and external design conditions of a typical building were selected and analyzed to determine the type of PCM, and the phase change temperature required for space cooling. Subsequently, a PCM of Plus-ice A17 was selected and charged into a small-scale finned-tube heat exchanger. Extensive measurements were conducted on the PCM heat exchanger at different operating conditions. Meanwhile, a three-dimensional computational fluid dynamics model for the PCM heat exchanger was developed and validated with the experimental measurements and thus simulated. When the airflow velocity increases from 1.3 m/s to 6 m/s, the phase change periods decrease by 25% and 13% for the PCM charging and discharging processes respectively. When the PCM thermal conductivity increases from 1 W/(m·K) to 8 W/(m·K), the phase change periods reduce by 36.3% and 47.7% for the PCM charging and discharging processes respectively. In addition, for the same increased range of PCM thermal conductivity, the charging energy efficiency increases by 16.3%, and the discharging energy ratio drops by 7.1%

Performance evaluation and optimal design of supermarket refrigeration systems with supermarket model "SuperSim", Part I: Model description and validation

This is the post-print version of the final paper published in International Journal of Refrigeration. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2010 Elsevier B.V.Conventional supermarket refrigeration systems are responsible for considerable CO2 emissions due to high energy consumption and large quantities of refrigerant leakage. In the effort to conserve energy and reduce environmental impacts, an efficient design tool for the analysis, evaluation and comparison of the performance of alternative system designs and controls is required. This paper provides a description of the modelling procedure employed in the supermarket simulation model ‘SuperSim’ for the simulation of the performance of centralised vapour compression refrigeration systems and their interaction with the building envelope and HVAC systems. The model which has been validated against data from a supermarket has been used for the comparison of R404A and CO2 refrigeration systems and the optimisation of the performance of transcritical CO2 systems. These results are presented in Part II of the paper.DEFR

Bubble dissolution in horizontal turbulent bubbly flow in domestic central heating system

In a domestic central heating system, the phenomenon of microbubble nucleation and detachment on the surface of a boiler heat exchanger finds its origins in the high surface temperature of the wall and consequential localised super saturation conditions. If the surrounding bulk fluid is at under-saturated conditions, then after exiting the boiler, the occurrence is followed by bubbly flow and bubble dissolution. A comprehensive understanding of the fundamentals of bubble dissolution in such a domestic wet central heating system is essential for an enhanced deaeration technique that would consequently improve system performance. In this paper, the bubble dissolution rate along a horizontal pipe was investigated experimentally at different operating conditions in a purpose built test rig of a standard domestic central heating system. A high speed camera was used to measure the bubble size at different depths of focal plane using two square sectioned sight glasses at two stations, spaced 2.2 m apart. A dynamic model for bubble dissolution in horizontal bubbly flow has been developed and compared with experimental data. The effects of several important operating and structural parameters such as saturation ratio, velocity, temperature, pressure of the bulk liquid flow, initial bubble size and pipe inside diameter on the bubble dissolution were thus examined using the model. This model provides a useful tool for understanding bubble behaviours in central heating systems and optimising the system efficiency

Design Optimisation of CO2 Gas Cooler/Condenser in a Refrigeration System

AbstractAs a natural working fluid, CO2 has been widely applied in refrigeration and heat pump systems where heat is conventionally rereleased to ambient through external airflow. Owing to extraordinary thermophysical properties, especially its low critical temperature, the CO2 heat release through a high-pressure side heat exchanger will inevitably undergo either supercritical or subcritical processes, depending on ambient air temperatures and head pressure controls. Correspondingly, the heat exchanger will act intermittently as either a gas cooler or condenser within the system during an annual operation. Such evidence should therefore be taken into account for an optimal design of the heat exchanger and head pressure controls in order to significantly enhance the performance of both components and the associated system.To achieve these targets, two CO2 finned-tube gas coolers/condensers with different structural designs and controls have been purposely built, instrumented and connected with an existing test rig of a CO2 booster refrigeration system. Consequently, the performance of the CO2 gas coolers/condensers with different structure designs, controls and system integration at different operating conditions can be thoroughly investigated through experimentation. In the meantime, models of the finned-tube CO2 gas coolers/condensers have been developed using both the distributed (detailed model) and lumped (simple model) methods. The former is employed to give a detailed prediction of the working fluid temperature profiles, localized heat transfer rates and effects of pipe circuitry arrangements, while the latter is suitable for the simulation and optimisation of system integration with less computation time. Both models have been validated with measurements, and moreover the simple model has been integrated with other component models so as to create a system model. The effects of the CO2 gas cooler/condenser sizes and controls on the system performance can thus be compared and analysed

Comparative tests on the performance of solar stills enhanced by pebbles, corrugated plate and membrane distillation and construction of performance prediction model for rock type still

To improve the water production capacity of solar still (SS), realize the theoretical prediction of the enhanced SS performance, and enrich the theoretical research basis of the desalination technology of SS, this paper sets up three kinds of enhancement measures, namely, rock, corrugated plate and membrane distillation, tests the enhanced water production effect, and reveals the enhanced operation mechanism. At the same time, a per- formance prediction model of rock enhanced was established based on the body-centered cubic stacking rock technology, and the influence of rock parameters on the distillation effect was studied. The study found that the water production increment of the three enhancement measures was concentrated in the rising period of the water production of the SS, and the total water production was 6.38 %, 12.30 % and 11.63 % higher than that of the traditional basin SS, respectively. The rock or corrugated plate enhances the distillation effect by elevating the seawater temperature and its temperature difference with the cover plate, and the membrane distillation increases the total water production through the additional water production of the membrane. Moreover, the constructed model can effectively predict the characteristics of rock enhanced SS. The increase in the rock layer thickness and the decrease in the rock particle size and material heat capacity both enhance the total daily water production, but the effect of rock particle size and material is weak. Although the increase of thickness increases the water production obviously, it aggravates the fluctuation of water production