Applied Thermal Engineering最新文献

The core design strategy for minimizing CO₂ emissions in gas power plant entails combining a spray ejector condenser (SEC) and separator to accomplish steam condensation and CO₂ purification. This innovative process involves direct-contact condensation of steam with CO₂, facilitated by interaction with a subcooled water spray, along with a cyclone separator mechanism intended for generating pure CO₂. The investigation of the SEC section, both experimentally and analytically, provides crucial insights into its operational dynamics. Given the susceptibility of cyclone efficiency to fluctuations in SEC conditions, this research endeavors to examine the impacts of CO₂ volumetric flow rate and droplet break-up within the SEC on the separation efficacy of the cyclone separator. Additionally, the impact of cone size on the performance of the cyclone has been investigated. Here, a three-dimensional, transient, and turbulent cyclone separator is numerically simulated using Ansys Fluent 2021 R1. The Reynolds Stress Model is employed to simulate turbulent flow, while a mixture model is utilized to replicate swirl two-phase flow within the separators. The findings revealed that reductions in steam and CO₂ flow rates were associated with a decrease in outlet temperature but an increase in SEC inlet temperature, leading to a rise in temperature difference and heat transfer rate. Furthermore, an augmentation in cyclone cone size (from 0.2 to 0.5 m) resulted in enhanced separation efficiency (from 77.30 % to 80.98 %) alongside an elevation in pressure drop (from 6.08 Pa to 10.91 Pa), suggesting a compromise between CO₂ purification and energy consumption. Additionally, elevated CO₂ flow rates induced a rise in pressure drop and separation efficiency, ultimately achieving maximum efficiency at a rate of 24 $\frac{g}{s}$. Moreover, the exploration into droplet breakup manifesting in a boost in separation efficiency from 50.98 % to 100 % across droplet diameters ranging from 1 to 20 $\mu m$.

With the development of market-oriented reformation, integrated energy system with high proportion of new energy installed capacity are gradually facing the pressure of power fluctuation stabilizing. This paper exploits the weak equilibrium characteristic of thermal system to solve high frequency fluctuation of new energy sources. It establishes a multi-frequency stable operation optimization model of integrated energy system considering the virtual energy storage characteristics of thermal network. Firstly, the model exploits the wavelet packet decomposition method for the frequency decomposition of the fluctuation stabilizing demand faced by the integrated energy system. Secondly, this study investigates the technical characteristics of source-load-storage equipment regulation and heat network temperature-quantity regulation in the thermal system. Finally, considering the equipment life and system fluctuation smoothing needs, an integrated energy system stability optimization model is established. Meanwhile, a modified honey badger algorithm is proposed to realize the case optimization simulation.The result shows that the total operating cost of the system is reduced by 8.45%. As the thermal system regulation replaces the high-frequency regulation function of the energy storage equipment, the service life of battery increased by 67.6%. Therefore, the model effectively solves the integrated energy system fluctuation stability problem with high proportion of new energy installed capacity.

The Ca(OH)₂/CaO reaction system presents broad development prospects for thermochemical heat storage. However, the poor thermal conductivity of heat storage material and the complex internal flow limit the performance of the reactor. To improve the performance of the reactor, this work designs a new improved fixed bed reactor with baffle. The comparison with and without baffle, as well as the effects of operation conditions and structural parameters of the improved reactor on heat storage and release performance, are numerically investigated. The results show that the improved reactor can enhance heat transfer and flow inside the reactor, resulting in performance improvement. After baffle installation, there is a 64% decrease in the pressure drop, a 29.14% reduction in the heat storage time, and a 35.45% reduction in the heat release time. Besides that, the improved reactor can maintain the high temperature for longer at the outlet during heat release. The orthogonal test design analysis reveals that the inlet velocity of direct heat transfer fluid has the greatest impact on heat storage, while the inlet velocity of indirect heat transfer fluid has the smallest impact, and the same applies to heat release. The thermal conductivity and permeability of the porous baffle have minor impacts, but the air distribution chamber has a noticeable influence. This work can guide the optimization design and operation adjustment of the reactor.

The characteristics of two-phase flow distribution from a vertically installed annular refrigerant distribution header to multiple branch ports are investigated, with a specific focus on simulating a real variable refrigerant flow heat pump (VRF HP) system. A test section is fabricated to replicate an actual distribution header for empirical investigation, utilizing an air–water mixture as the working fluid. A series of experiments are conducted to examine distribution characteristics, involving variations in the location of the main inlet port, operational mode, total mass flow rate, and void fraction. The experimental results indicate that the two-phase mixture exhibits a relatively uniform liquid phase distribution to the branch ports when the main inlet port is positioned at the bottom side. However, maldistribution to branch ports is intensified as the inlet void fraction increases and the flow rate of the two-phase mixture decreases. Furthermore, a series of computational fluid dynamics analyses is performed to investigate the distribution characteristics of R410A, the actual working fluid utilized in VRF HP systems. These analyses are conducted under the operational conditions typical of VRF HP systems. Despite identical Reynolds numbers for the two-phase R410A flow and air–water mixture flow, the former exhibits a notably uneven distribution to branch ports owing to the distinct thermophysical properties, particularly density and viscosity. The findings from this investigation enhance the comprehension of two-phase flow distribution characteristics in a vertically installed refrigerant distribution header under real VRF HP system conditions, offering valuable insights into mitigating the maldistribution of refrigerant flow to branch ports.

Vacuum Membrane Distillation stands out as a solar desalination method utilizing membrane pressure differentials to evaporate seawater and collect it as fresh water. On the other hand, the new design of compact (or integrated) solar water heaters can be very suitable for Vacuum Membrane Distillation use. In the new design of solar system, its upper part, which is under buoyancy pressure, can have a very suitable space for installing the Vacuum Membrane Distillation system. The purpose of this research is to simulate the proposed design and a dynamic time-domain model was developed for comprehensive simulation, encompassing absorber plate characteristics, collector dimensions, mass flow rate, vacuum pressure reduction, the influence of increasing membrane rows, and seawater concentration. Simulation results showcase the system’s potential, yielding $57.12kg/m^2/day$ of freshwater daily with an average gain output ratio exceeding 1.1. The estimated water production cost is $0.039USD.L^{-1}$, along with the generation of $4.3kg.kg/m^2/day$ of chlorine and $0.037g.kg/m^2/day$ of bromine from Mediterranean seawater. For super saline water, the projections indicate a daily production of $104.46kg.kg/m^2/day$ of freshwater with a production cost of less than 0.01 USD/L, a GOR of 1.7, and a daily production of $37.8kg.kg/m^2/day$ of chlorine for a collector area of 6 m². These outcomes underscore the efficiency and economic viability of the proposed design, positioning it as a promising solution for seawater desalination challenges.

Considering the energy needs of developing countries, thermal power plant facilities are necessary for power generation. However, usable thermal energy is lost because the necessary technologies to exploit the waste heat from industrial processes have not yet been implemented. In this sense, the present analysis shows the experimental study of a prototype Steam Rankine Cycle energy system. The prototype uses the residual heat of the combustion gases from one of the generators installed in a thermal power plant as a heat source, the maximum temperature of these gases is 320 °C. This prototype is considered the first in the region to perform heat recovery with a power cycle with water as the working fluid. Considering the country’s economic constraints, the prototype was built using a 20 kW twin-screw compressor modified to function as a turbine. This prototype aims to increase the overall performance of the generating unit. For the characterization of this prototype plant, energy and exergetic efficiency analysis was carried out using the experimental data. The application showed that the measured efficiency of the twin-screw expander was low at 18.44 %. In contrast, the maximum expander work was 15.81 kW, the overall cycle efficiency was 3.52 %, and the exergetic efficiency was 5.45 %.

While the use of CO₂ as a natural refrigerant become widespread, there is a scarcity of research on refrigerant charge effects in transcritical CO₂ refrigeration applications, especially for systems with flash-tank and two-stage compressors. In this work, an experimental investigation of the effect of the Normalized Refrigerant Charge (NRC) on the parameters and performance of a Flash-Tank Vapor Injection (FTVI) CO₂ refrigeration system is carried out. Within the explored NRC range of −0.02 to 0.247, an optimal NRC of 0.18 was identified that yields a maximum COP regardless of operating conditions. Results suggest that typical operating parameters such as temperature and pressure are not being affected by the NRC because of the charge buffering function of the flash-tank. However, the liquid–vapor separation capacity of the flash-tank causes a reduction in the refrigeration capacity when the system is not charged properly. This refrigeration capacity detriment related to undercharged conditions becomes more severe under high ambient temperatures, with a maximum COP penalty of 20 % when comparing the lowest charge versus the optimal charge performances at a gas cooler outlet temperature of 40 °C.

Thermosyphons that operate with low evaporator heat fluxes and exhibit a small temperature difference between the evaporator and condenser have significant potential for thermal management of buildings or other spaces when used in conjunction with radiative sky cooling. In such applications, thermosyphons may be used to transfer heat from the evaporator, located in the space to be cooled, to the condenser which may be in contact with a radiator covered with spectrally selective radiative sky cooling coating. To ensure adequate cooling of the space, the temperature difference between the evaporator and condenser in such a thermosyphon should be as small as possible and the thermosyphon should startup under these conditions. For such applications, this paper presents an experimental investigation of a tubular thermosyphon with a horizontal evaporator and inclined condenser. For a given external geometry of the thermosyphon, this study presents an experimental investigation of five copper thermosyphons with a variety of working fluids, fill ratios and evaporator inner threading. Of these thermosyphons, one contains methanol with an evaporator fill ratio of 50% and the other four contained R134a. Two of the R134a thermosyphons had a fill ratio of 25% and the other two had a fill ratio of 50%. For each fill ratio, one of the thermosyphons had a smooth inner surface of the evaporator, whereas the other was circumferential threaded on the inside. The goal of the threading was to enhance wetting of the periphery of the evaporators since they were partially filled. The thermosyphons were tested in both evaporator and condenser-controlled modes to examine their start-up performance, thermal resistance, and cool-down capability. In the former, heat flux was varied at the evaporator end and the condenser was cooled, whereas, in the latter mode, the condenser temperature was varied, and the evaporator was dipped in a water tank (that acted as a thermal storage medium). In the evaporator-controlled mode, observations from experiments showed that the R134a thermosyphons required lower super-heat for startup and had significantly lower thermal resistance than methanol. Further, the threading of the inner surface of the evaporators significantly improved startup and further reduced the thermal resistance. During the condenser-controlled mode, the cool-down time of the thermal storage medium was shorter with the threaded R134a thermosyphons. The evaporator fill-ratios considered in this study did not appear to significantly affect the performance of the thermosyphon.

The current research explores the heat transfer and erosion phenomena in a tube employing newly designed inserts. Seven different geometries for the inserts have been developed into three dimensions. The fluid used is water with iron oxide nanoparticles and the flow regime is turbulent. The modeling technique involves a finite volume method with the Eulerian-Lagrangian framework incorporating a particle-scale erosion model. The outcomes indicate that raising the velocity and volume fraction of nanoadditives enhance both the heat transfer and erosion. In comparison to the scenario without fins, the heat transfer can be increased by approximately 8.07, 6.22, 18.03, 53.72, 45.21, 14.74 and 13.08 for the considered cases G1, G2, G3, G4, G5, G6 and G7, respectively. The specific geometric design of the inserts significantly influences the heat transfer and erosion in the tube, with geometry G4 showing the most substantial improvements in the heat transfer (approximately 53 %) and erosion (around 100 %) compared to a smooth tube. Factors such as velocity, particle size, fluid viscosity and insert shape affect the erosion in the tube with water demonstrating higher erosion rates than oil. The introduction of iron oxide nanoparticles leads to 2–3 times increment in the tube wall erosion. The erosion estimation on the tube wall has been performed using an adaptive neuro-fuzzy inference system artificial intelligence model with R² = 0.981.