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[en] Highlights: • Subcooled boiling first occurs in the inner side of helical coils under the studied test conditions. • ONB can appear in helical coils when mean wall temperature is lower than saturation temperature. • Effects of various parameters on wall temperature and HTC are studied in helical coils. • New correlations have been proposed for ONB and HTC of subcooled boiling in helical coils. - Abstract: Helical coils have been widely used in a variety of applications, such as heat recovery processes, power plants, cryogenic systems, etc, due to the practical importance of high efficiency heat transfer, compactness in structure, ease of manufacture and arrangement. Experiment investigation of heat transfer characteristics of subcooled flow boiling in helical coils with different inner diameters and coil diameters was performed in the present paper. The rise angles of these helical coils were all 6 degrees. The system pressure was in the range of 1.8 MPa and 7.8 MPa, mass flux ranged between 300 kg/(m2·s) and 1100 kg/(m2·s), and heat flux varied from 100 kW/m2 to 450 kW/m2. The experimental results showed that the onset of subcooled boiling was significantly influenced by heat flux and system pressure. A new correlation to predict the onset of subcooled boiling was proposed, correlating experimental results within ±20%. The effects of heat flux, mass flux and system pressure on heat transfer behavior in subcooled boiling region were discussed. A new correlation of subcooled boiling heat transfer coefficient in helical coils was developed, correlating experimental results within ±20%.
[en] Highlights: • The ULFM shows acceptable performance to predict CO for Sandia Flame D. • The ULFM shows good agreement with CO measurement at the HRSG stack. • Parametric study is performed on the influence of FCD and layout of activated burners in HRSG. • The CO emission is affected by the lean and rich limits for combustion of local mixture. - Abstract: Computational simulation is performed for flow field and carbon monoxide (CO) emission in an industrial scale heat recovery steam generator (HRSG) by ANSYS Fluent v13. The geometrical details are reproduced with burner holes and swirler blades simplified to avoid excessive computational burden. Turbulence-chemistry interaction is modeled by the steady laminar flamelet model (SLFM) and the unsteady laminar flamelet model (ULFM) through a lookup table without time consuming integration of stiff elementary reaction steps. The ULFM showed good agreement with measured CO mass fractions near the extinction limit for Sandia Flame D in Turbulent Nonpremixed Flame (TNF) Workshop. The proper trends of variation and the same order of magnitude of CO mass fractions were reproduced by the ULFM for the three reference cases of varying HRSG loads. Parametric investigations were performed to identify the factors influencing exhaust CO with respect to the number and layout of activated burners and flow correction device (FCD). Results showed two competing factors for CO emission, rich mixture by undermixing and lean mixture by overmixing, which may lead to local extinction below the flammability limit.
[en] Highlights: • An ORC modeling based on equivalent temperatures is described. • A reconstruction method of the thermodynamic cycle is presented. • The selection procedure of organic fluids based on various criteria is discussed. • A case study is presented for various source temperatures. - Abstract: Organic Rankine cycles (ORC) are part of heat recovery technologies, which allow thermal wastes to be converted into a mechanical power. These systems are suitable for various applications characterized by a large range of source and sink temperatures. Nonetheless, the selection of the organic working fluid is essential during ORC design processes; they have a major impact on the overall engine performance. Several works have been intending to develop appropriate methodologies to determine the optimal working fluid. Recently, an optimization approach independent of the organic fluid, has been proposed in the open literature, where optimal operating conditions are expressed in terms of equivalent temperatures and overall heat transfer surface areas. However, at the end, the primitive variables under which the real system must operate, should be determined. Within this framework, for a given working fluid, this paper presents a reconstruction method of a traditional thermodynamic cycle that allows actual thermodynamic variables (pressures, temperatures, mass flow rates) to be calculated. Furthermore, a methodology is also proposed for selecting the most appropriate organic working fluid, subjected to environmental and practical engineering design criteria. To emphasize the potential of the proposed approach, the entire procedure is then applied to a particular case study.
[en] Highlights: • The operating performance of the metering pump for different strokes was analyzed. • The influence of the metering pump on the ORC system was conducted. • The maximum value of the specific speed was determined. • The effect of pump efficiency on net output power was discussed. - Abstract: In this paper, a performance test of a hydraulic diaphragm metering pump using R123 is conducted. The interaction relationships of key parameters of the pump and its influence on the performance of the ORC system are analyzed. The application feasibility of the pump has been proved by comparison to previous studies. The results indicate that the mass flow rate of the pump varies from 0.23 t/h to 2.06 t/h and is mostly independent of outlet pressure. Both power input and actual efficiency of the pump increase with stroke; the actual efficiency reaches up to a maximum of 88.27%. The specific speed decreases with the increase of outlet pressure. Moreover, the actual net power output and thermal efficiency of the ORC system increase with evaporating temperature. Actual net power output is more sensitive than thermal efficiency to stroke. The thermal efficiency presents a nonlinearly rapid decline trend with the increase of specific speed. Back work ratio (BWR) can reach up to a maximum of 0.93. Thus, the power input of working fluid pump is not negligible in the ORC system and the assumptions of actual efficiency of pump should be dependent on experimental results according to various operating conditions.
[en] This study focused on the evaluation of the stress and fatigue life of the high-pressure evaporator of a heat recovery steam generator (HRSG). First, the piping system was analyzed, and the tube bundles were shown to satisfy the acceptance criteria under sustained and cyclic loads as per ASME B31.1. Next, an outlet header, a representative thick component, was evaluated as per the ASME Boiler and Pressure Vessel Code. In analyzing the header, the nozzle loads resulting from the analysis of the piping system were applied. As a result, the stress of the header under the design condition was found to satisfy the acceptance criteria. Finally, the temperature and thermal stress of the header during the transient operating conditions were analyzed, and the fatigue life at the nozzle and tube bores were evaluated. The header was found to be safe from fatigue failure.
[en] Highlights: • Heat recovery efficiency of MVHR plays a decisive role in determining air preheaters. • Maximum air supply temperature increase of 25% and 41% were found. • A reduction of ventilation heating load by 27% and 40% were found. • The proposed system reaches its highest contributions when η of MVHR is 80%–85%. - Abstract: The current study evaluated the potential of reducing ventilation heat load by using heat from waste water in mechanical ventilation with heat recovery (MVHR) served Swedish residential buildings. A typical Swedish low-energy, multi-family house locating at the northern part of Sweden was selected to present the analysis. The building was locating at the northern part of Sweden and was served by mechanical ventilation with heat recovery (MVHR). The data from on-site measurements and analytical model were applied to evaluate the reduction potential of the suggested heat recovery system. The study focused on the evaluation of benefit of using an air preheater in front of the existing MVHR system. Two different sizes of an air preheater design: Small air preheater with the size of 0.4 m × 0.4 m × 0.4 m (), feed with waste water flow of 0.15 kg/s (from storage tank to air preheater); and a large air preheater with the size of 0.8 m × 0.8 m × 0.4 m (), feed with waste water flow of 0.2 kg/s. It was found that the heat recovery efficiency of MVHR is the core to determine the selection of air preheaters. In comparison to the MVHR without air preheater, maximum air supply temperature improvements of 25% and 41% were found from and , respectively. The studied system reached its highest contributions when the heat recovery efficiency of MVHR was between 80% and 85%. On average, can reduced the peak heat load up to 27%. can reduce the peak heat load up to 40% in the studied climate.
[en] The majority of biogas systems uses block heat and power plants to utilize the biogas. The biogas is converted to heat and electric energy in these system parts. A part of both energy shares is necessary to operate the biogas system. Heat consumptions and feeding protocols of 4 agricultural systems spanning up to 10 years could be used for the study regarding the reduction of the infed heat quantity. Furthermore, the temperature of the fermentation substrate and the biogas were logged in 6 biogas systems in a weekly cycle over a period of 15 months. Aside from environment-related temperature fluctuations of the substrate, a not insignificant cooling of the silage could be determined in the reception bunkers and the screw conveyors. Heat quantities for the heating of the substrate were determined based on the temperature measured, which are maximally 81% of the infed heat in the winter months for liquid manure and up to 60% for maize. A heat transfer from the fermentation residues to the liquid manure can be easily implemented as both substrates can be pumped. Depending on the design of the heat exchanger, the heat necessary to achieve the process temperature can be reduced significantly for the liquid manure and no external heat would be necessary in an optimum case scenario. The cooling of the maize silage in the solid storage and in the screw conveyors can be reduced with coverings and insulation. Increasing the inlet temperature by 10 °C would reduce the heat requirement by 25%.
[en] Highlights: • The influence of disk configurations on droplets characteristics is investigated. • The droplets size, droplets size distributions, filament mass fraction and particle uniformity were analyzed. • A correlation of dm for four types of disks is presented. • The majority of the droplets size is populated in the range of 0.2–1.0 mm. • The flat/arc-edge disks represent a superior uniformity compared with the other disks. - Abstract: The influence of rotary disk configurations on droplets characteristics in molten slag ligament granulation was investigated by using the high-speed camera visualized system. Rosin/Paraffin mixture was adopted as the analogue of the molten slag according to the similarity theory. Four groups of disks with different configurations were studied. The droplets characteristics, such as droplets size, droplets size distributions, filament mass fraction and particle uniformity were analyzed. Generally, the droplets mean diameters (dm) decreases by increasing the rotary speed (ω) or disk diameter (D), and increases slightly by increasing the liquid flow rate (Q). Based on the experimental data, a simple correlation of dm for four types of disks is presented. The majority of the droplets size is populated in the range of 0.2–1.0 mm for all ω and Q, which accounts for over 70% mass fraction. The ω has significant influence on droplets size distribution and particle uniformity. The flat/arc-edge disk configurations can contribute to produce the narrower size range of droplets and obtain the superior particle uniformity compared to the other two types of disks. In the ligament breakup mode, the filament mass fraction reduces by increasing Q or decreasing ω. Meanwhile, the liquid film breakup mode also has significant impact on the formation of filament.
[en] Highlights: • A SINDA/FLUINT simulation model of a TPCT charged with R245fa was established. • Good agreement was achieved between simulation model and experimental results. • The effects of relevant factors were investigated on performances of the TPCT. • Performances of the large scale TPCTs were investigated by the developed model. - Abstract: Two-phase closed thermosyphons (TPCTs) are simple and efficient heat exchangers. They have been explored for use in the renewable energy resource utilization marker and low grade thermal energy heat recovery systems. A transient simulation model for a TPCT was established by SINDA/FLUINT with low global warming potential Freon R245fa as working fluid. The TPCT was manufactured from a 40 mm inner diameter (Di), 3 m long smooth copper tube with a wall thickness of 2 mm. It consists of the evaporator, adiabatic and condenser sections with 1 m long for each section. The evaporator section was immersed in a water bath and the condenser section was cooled by recycled water. The effects of water bath temperature Tb and inlet temperature of cooling water Tcw,i were investigated by experimental and simulation methods. The results show the heat transfer rate Q and overall heat transfer coefficient U increase with the increase of Tb, and the decrease of Tcw,i. Good agreement between experimental and simulation results confirms the model is accurate and reliable. The influence of filling ratio (FR) and Di on the performance of TPCT was also investigated. The optimum FR for Di of 30 mm, 40 mm and 50 mm are 15%, 15% and 25% respectively. Moreover performances of 60–150 m long TPCTs were investigated based on the developed model.