24,127 research outputs found
Numerical modelling and performance maps of a printed circuit heat exchanger for use as recuperator in supercritical CO2 power cycles
No full textIn heat to power systems with CO2 as working fluid in the supercritical state (sCO2), heat exchangers account for nearly 80% of the capital expenditure. Therefore, improved design, materials and manufacturing methodologies are required to enable the economic feasibility of the sCO2 technology. In this study, a comparison of different modelling methodologies for Printed Circuit Heat Exchangers (PCHE) is proposed to identify strengths and weaknesses of both the approaches. The elementary heat transfer unit of a PCHE recuperator for sCO2 applications is firstly modelled using 1D and 3D CFD methodologies respectively; implemented in GT-SUITE and ANSYS FLUENT software. After the comparison in terms of heat transfer performance and pressure drops, the 1D approach is used to model a 630kW PCHE recuperator. The PCHE model calibration on the design point, followed by its validation against off-design operating points provided by the manufacturer, eventually enabled to broaden the simulation spectrum and retrieve performance maps of the device. The CFD models comparison shows a good agreement between temperature profiles. However, the local heat transfer coefficient, modelled in the 1D approach through the Dittus-Boelter correlation, experiences a +10% offset on the hot side and a -20% on the cold one with respect to the 3D CFD calculations. Besides, the performance maps of the full scale PCHE recuperator show that the maximum temperature of the hot stream impose a greater influence than the maximum pressure of the cold one in terms of overall heat transfer coefficient. Nonetheless, both these operating parameters contribute to affect the heat exchanger effectiveness.The Engineering and Physical Sciences Research Council (EPSRC) of the UK and the European Union’s Horizon 2020 research and innovation programme
Investigation of the entrainment and infiltration rates through air curtains of open low-front refrigerated display cabinets
No full textThis thesis was submitted for the degree of Doctor of Philosophy and was awarded by Brunel UniversityThe high energy demand associated with open multi-deck refrigerated display cabinets is a direct consequence of their open design. The interaction between the cold refrigerated air inside the cabinet and the relatively warm air of the supermarket takes place across the air curtain, which serves as a non-physical barrier between the customers and the products. It has been estimated that 70% to 80% of the cabinet’s cooling load is due to ambient air infiltration into the cabinet refrigeration apparatus, which was previously entrained through the descending air curtain. A new generation of display cabinets has immerged in recent years, where the display-to-floor area has increased for the sake of maximizing sales. This modification leaves the air curtain with a larger display opening to seal against. Therefore, the design of such cabinets has now become more challenging, especially when attempting to ensure product integrity and temperature homogeneity while attempting to minimize their energy consumption.
In this work, advanced numerical and experimental techniques have been integrated to quantify and also minimize the entrainment rate through the air curtain and the infiltration rate into open low-front refrigerated display cabinets. Experimentally, the Particle Image Velocimetry (PIV) technique has been used to map the velocity profile along the air curtain while the Infrared (IR) Thermography technique has been used to map the temperature profile across the cabinet. The Computational Fluid Dynamics (CFD) technique has been used in both case and parametric studies after confirming its validation with experiment. CFD was found to be a valuable tool for the simulation of open low-front refrigerated display cabinets, and the credibility of the results was assured when the boundary conditions were fine-tuned by experimental data.
This thesis has demonstrated a systematic procedure where the entrainment rate through the air curtain can be quantified. The effect of various Discharge Air Grille (DAG) parameters was studied, and it was found that the entrainment rate is highly sensitive to the velocity profile and magnitude at the DAG. A velocity profile with a ramp shape having the maximum velocity near the cabinet yielded the minimum entrainment rate, hence the cabinet cooling load was reduced. In addition, two techniques were introduced for the determination of the infiltration rate of the cabinet. The first utilises the tracer-gas method to determine the specific amounts of ambient dry air and water vapour entering the evaporator coil, and the second uses psychrometrics to quantify the infiltration load as well as the other cooling load components by identifying the various heat transfer processes encountered during the operation of the cabinet. The ambient air infiltrated into the cabinet, although corresponds to 31% of the total mass flow rate, was found to be responsible for at least 85% of the total cooling load of the cabinet. This indicates that low-front cabinet suffer more from infiltration.
The contribution of this work is by providing a better understanding towards the entrainment and infiltration processes related to open refrigerated display cabinets. The new techniques introduced in this work can help designers to better assess the impact of different design parameters and quantify the amounts of the entrainment and infiltration rates associated with open low-front refrigerated display cabinets
Investigation into the design and optimisation of multideck refrigerated display cases
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The refrigeration energy load in a modern day supermarket makes up a large proportion of the total energy bill. Better design of refrigerated display cases would reduce this load and also have a corresponding effect on the running costs of the refrigeration plant. Further enhancements such as the reduction of air overspill from the case would also influence the aisle temperatures and therefore the comfort levels in the store. This research project uses the technique of computational fluid dynamics (CFD) to investigate the contemporary design of a vertical multideck refrigerated display case. From a two dimensional computational model conclusions were drawn as to the principles of operation of the case. During the course of the project, a custom designed experimental facility was constructed, capable of testing the display case according to the relevant test standards. Using this facility, experimental validation was carried on a number of the design modifications to assess the actual refrigeration load against that predicted by the CFD model. The success of this validation allowed further work into the feasibility of certain design changes by making modifications to the CFD model. The work presented in this thesis makes a contribution to the global effort towards the reduction of the energy consumption by retail refrigeration systems. It does this by showing that possibilities do exist for an improvement in the energy efficiency of multideck refrigerated display cases and that CFD provides a useful tool towards this goal. It also demonstrates the design modifications which proved to yield a saving in energy. These were a reduction in the mass flow rate of air around the case, the inclusion of a honeycomb section on the air curtain outlet of the case the addition of a front upstand and the introduction of a second air curtain thus applying a velocity gradient across the curtain.This work was supported by the Engineering and Physical Sciences Research Council and Safeway Stores Plc
Evaluation of the energy impact of PCM tiles in an Airport Terminal Departure hall
No full textCopyright @ 2013 CIBSEIn most past studies, passive PCM (phase change materials) systems have been tested for relatively small office spaces where the airflow is of minimal consequence in the overall energy consumption of the space. This paper on the other hand, reports on the application of PCM tiles on the floor of an Airport terminal space, similar to London Heathrow Terminal 5 departure hall, where in such large open spaces, the influence of airflow is crucial for the evaluation of the energy performance of AC units. In this paper, the evaluation of the energy performance of PCM tiles is obtained through a coupled simulation of TRNSYS and CFD. TRNSYS simulates the AC unit and PID control systems, while CFD is used to simulate the airflow and radiation inside the terminal space. The phase change process is simulated in CFD using an in-house developed model which considers hysteresis effects and the non-linear enthalpy-temperature relationship of PCMs. Although, a displacement ventilation (DV) system is actually employed at Heathrow Terminal 5, this study also compares the performance of the PCM tiles for a mixed ventilation (MV) system. Due to large computing times associated with CFD, discrete time-dependent scenarios under different UK weather conditions are used. The yearly energy demand is then determined through the heating/cooling degree day concept using base temperatures of 18 and 23 °C for HDD and CDD, respectively, similar to the comfort temperature range in the Terminal. The results show that the use of PCM tiles on the floor of the Terminal departure hall can lead to annual energy savings of around 3% for the DV system and 6% for the MV system, corresponding to 174 MWh/year and 379 MWh/year for the Terminal building.This work was funded by the UK Engineering and Physical Sciences Research Council (EPSRC), Grant No: EP/H004181/1
Inventory control assessment for small scale sCO<inf>2</inf> heat to power conversion systems
No full textData availability statement: Data related to the paper and other information relating to the paper can be obtained by contacting the corresponding author.The control of the main cycle parameters in supercritical CO2 (sCO2) systems during off-design and transient operation is crucial for advancing their technological readiness level. In smaller scale power units (<0.5–5 MW), costs and complexity constraints limit the number of auxiliary components in the power loop, making the design of the control system even more challenging.
Among the possible strategies, the regulation of system inventory, which consists in varying the CO2 fluid mass in the power loop to achieve a given control target, represents a promising alternative. Such technique however poses several technical challenges that are still to be fully understood. To fill this gap, this work presents a comprehensive steady-state and transient analysis of inventory control systems, referring in particular to a 50 kW sCO2 test facility being commissioned at Brunel University.
Stability implications (e.g. pressure gradients in the loop) and the effects of variable inventory tank size are discussed. Tank volumes 3 times higher than the one of the power loop (including the receiver) can lead to a higher controllability range (±30% of the nominal turbine inlet temperature) and an extended availability of the control action (slower tank discharge). A PI controller is also designed to regulate the turbine inlet temperature around the target of 465 °C in response to waste heat variations.European Union's Horizon 2020 research and innovation program under grant agreement No. 680599 for the I-ThERM project; EPSRC Grant No. EP/P004636 for the OPTEMIN project and Grant No, EP/V001795/1 for the SCOTWAHR project
Investigation of heat transfer in metal-foam tubes
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Nowadays metallic foams attract increasing attention in making compact heat exchangers due to its advanced thermal properties. In this project, the performance of forced convection and flow boiling heat transfer in metal-foam tubes has been investigated experimentally and theoretically. Compared to those of plain tubes, the research results reveal that the use of metal foam can significantly enhance heat transfer capabilities due to its strong fluid mixing capability and the high surfacearea density. In present study, the heat transfer performance and flow resistance of R134a vapour flow in metal-foam tubes have been measured. The results show that both the microstructure of metal foams and tube/foam attaching methods (contact thermal resistance) can significantly affect the heat transfer performance. Compared to plain tubes, the use of metal foams could improve the heat transfer by 5-15 times. To examine the effect of different tube/foam combining methods on heat transfer, a group of metal-foam tubes with different attaching methods have been tested experimentally. Since no experimental result for two-phase heat transfer in metal foams can be found in the open literature despite their potential wide range applications, the boiling flow and heat transfer in horizontal metal-foam tubes are experimentally investigated. The results show that the two-phase flow resistance and heat transfer both increase as the pore size tends to be smaller for a given porosity. The boiling heat transfer will be enhanced by increasing the vapour quality for high mass flow rates, but it is not always true for low mass flow rates. This different heat transfer behaviour can be attributed to different flow patterns occurring inside the metal-foam tubes. The two phase flow pattern can be indirectly inferred from the cross-sectional wall surface temperature fluctuations and the temperature difference between the wall surface and refrigerant fluid. In addition to the experimental research, analytical and numerical investigations have been conducted to predict the heat transfer performance of forced convection and flow boiling heat transfer in these tubes. The effect of contact thermal resistance between the tube wall and metal foam structures was considered in the model. The results show that the overall thermal resistance of a metal-foam tube is a combination of the resistance of the metal-foam structure and the resistance between solid phase and fluid phase. The increase of relative density and the decrease of pore size of metal foams can reduce these resistances respectively. The thesis also reports the investigations of the effects of these parameters on enhancing the overall heat transfer performance. This was carried out through a detailed parametric analysis. The results show that the thermal performance of a metal-foam heat exchanger can be superior to that of conventional finned tube heat exchangers.Funding was obtained from the UK Engineering and Physical Sciences Research Council
(EPSRC grant number: GR1T24364/01) to provide fund for the project, Brunel
University (Brief Award, WAE-DPA301)
Dynamics of SCO2 heat to power units equipped with dual tank inventory control system
No full textCopyright © 2021 The Author(s). A key aspect in upscaling the technology readiness level of supercritical CO2 (sCO2) power generation systems is the control of the main cycle parameters (i.e. temperature at the turbine or compressor inlet) at off-design conditions and during transient operation. A further challenge in small scale (<0.5MWe) systems is the limited number of control variables due to the streamlined configuration of the power units. Among the possible control strategies, is the regulation of the system inventory, which consists of the variation of the CO2 fluid mass (or charge) in the power loop to achieve a given control target. Such strategy, which relies on different storage tanks for injections/withdrawals of the working fluid into/from the system, poses several technical challenges that are still not fully understood. To fill the gap, this work presents an analysis of inventory control systems. The impact of this control approach is investigated using a high-fidelity one-dimensional simulation platform calibrated on a 50 kW simple regenerative High Temperature Heat to Power sCO2 test facility being commissioned at Brunel University London. Transient simulations are carried out to assess the dynamics of the main thermodynamic variables in the power loop and the inventory tanks. Stability implications (e.g. pressure gradients in the loop) as well as the effects of size of the inventory tanks are discussed. Inventory tanks with a volume 3 times higher than the one of the power loop (including the receiver) can lead to a higher controllability range (±30% of the nominal turbine inlet temperature) and an extended availability of the control action (slower tank discharge).European Union’s Horizon 2020 research and innovation program under grant agreement No. 680599; EPSRC (UK) research grants (i) EP/V001752/1; (ii) EP/P004636/1; (iii) EP/K011820/1
Performance evaluation and optimal design of supermarket refrigeration systems with supermarket model "SuperSim", Part I: Model description and validation
No full textThis 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
Techno-economic assessment of Joule-Brayton cycle architectures for heat to power conversion from high-grade heat sources using CO2 in the supercritical state
No full textBottoming thermodynamic power cycles using supercritical carbon dioxide (sCO2) are a promising technology to exploit high temperature waste heat sources. CO2 is a non-flammable and thermally stable compound, and due to
its favourable thermophysical properties in the supercritical state, it can achieve high cycle efficiencies and a substantial reduction in size and cost compared to alternative heat to power conversion technologies. Eight variants of the sCO2 Joule-Brayton cycle have been investigated. Cycle modelling and sensitivity analysis identified the Turbine Inlet Temperature (TIT) as the most influencing variable on cycle performance, with reference to a heat source gas flow rate of 1.0 kg/s and 650°C. Energy, exergy and costs metrics for different cycle layouts have been are compared for varying TIT in the range between 250°C and 600°C. The analysis has shown that the most complex sCO2 cycle configurations lead to higher overall efficiency and net power output but also to higher investment costs. Conversely, more basic architectures, such as the simple regenerative cycle, with a TIT of 425°C, would be able to achieve an overall efficiency of 25.2%, power output of 93.7 kWe and a payback period of less than two years
Effectiveness of CFD simulation for the performance prediction of phase change building boards in the thermal environment control of indoor spaces
No full textThis is the post-print version of the Article. The official published version can be accessed from the link below - Copyright @ 2013 ElsevierThis paper reports on a validation study of CFD models used to predict the effect of PCM clay boards on the control of indoor environments, in ventilated and non-ventilated situations. Unlike multi-zonal models, CFD is important in situations where localised properties are essential such as in buildings with complex and large geometries. The employed phase change model considers temperature/enthalpy hysteresis and varying enthalpy-temperature characteristics to more accurately simulate the phase change behaviour of the PCM boards compared to the standard default modelling approach in the commercial CFD codes. Successful validation was obtained with a mean error of 1.0 K relative to experimental data, and the results show that in addition to providing satisfactory quantitative results, CFD also provides qualitative results which are useful in the effective design of indoor thermal environment control systems utilising PCM. These results include: i) temperature and air flow distribution within the space resulting from the use of PCM boards and different night ventilation rates; ii) the fraction of PCM experiencing phase change and is effective in the control of the indoor thermal environment, enabling optimisation of the location of the boards; and iii) the energy impact of PCM boards and adequate ventilation configurations for effective night charging.This work was funded through sponsorship from the UK Engineering and Physical Sciences Research Council (EPSRC), Grant No: EP/H004181/1
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