Hasil untuk "Low temperature engineering. Cryogenic engineering. Refrigeration"

Jyoti Singh, Mayank Maheshwari, Keerthi Kumar B et al.

Abstract Currently, increasing the efficiency of power generation cycles is not the only goal for engineers; the focus is also on how it is achieved, whether through conventional or non-conventional energy sources. In conventional systems, changing the working fluid in the bottoming cycle has attracted engineers’ interest to boost cycle efficiency. Among various working fluids, carbon dioxide and ammonia water mixtures show promising thermodynamic properties that enhance both first and second law efficiencies. This research explores the use of transcritical carbon dioxide as the working fluid in the bottoming cycle of a combined cycle power plant with reheat cycles. The results indicate that, under operating conditions such as a topping cycle pressure ratio of 20, an ambient temperature of 303 K, and a turbine inlet temperature of 2000 K, the system performs better, with first law and second law efficiencies reaching 44.8% and 56.83%, respectively. Additionally, the maximum cooling water mass flow rate, observed at a condenser pressure of 0.9 bar, is 1.67 kg/s.

Zhang Bowen, Zhang Zhiqiang, Zhao Hongxia et al.

Quantitative research on the industrial application of direct cooling ice makers is limited, resulting in a lack of clarity in control mechanisms and inadequate heat transfer capability and uniformity in ice making. A mathematical model focusing on the refrigerant side of the ice mold evaporator was established, and a MATLAB simulation model was used to analyze the changes in heat transfer and flow parameters in the flow direction throughout the ice-making process, with comparisons drawn between the experimental data and the simulation results. The heat transfer rate before water icing was approximately 30% higher than that after water icing, and the refrigerant flow rates were significantly different. The heat flux in the superheat region decreased by 40.9% compared to that in the two-phase region, and reducing the superheat section can significantly enhance heat transfer and improve temperature uniformity. The thermal resistances of the water and ice sides accounted for 93.4% and 91.7% of the total resistance, respectively. Thus, the heat transfer of the water side or ice side should first be improved to optimize heat transfer. The simulation model can predict the change in the flow rate and simulate the superheat section, which provides a theoretical basis and practical guidance for the design and operation control of an ice-making machine and helps to improve the product performance and accelerate the ice-making process.

Zhang Ye, Sun Zhigao, Liu Xing et al.

Ice slurry has been widely used in air conditioners, food, and other industries owing to its good performance in flow, heat transfer and cold storage. To study the effect of solid particle size on the rheological characteristics of ice slurry, three kinds of polyethylene particles (density about 0.922 g/cm3) with average particle sizes of 0.31 mm, 0.43 mm, and 0.51 mm were selected as the solid phase in this study. The pressure drop of the mixed slurry flowing in a horizontal circular pipe was measured by changing its velocity and solid-phase content. A piecewise fitting rheological equation was determined based on the relationship between shear stress and shear rate. The effect of particle size on the rheological properties of two–phase liquid such as ice slurry was studied. The results show that the shear stress of the mixed slurry increases with increasing particle size, solid-phase content, and shear rate. The slurry behaves as a shear-thickening fluid, and its rheological index is always greater than 1. The rheological coefficient of the slurry increases with an increase in the solid-phase content under the condition of constant particle size. When the solid content is lower than 15%, the rheological coefficient increases significantly with increasing particle size.

Liu Li, Li Liansheng, Yang Qichao et al.

A thermodynamic analysis was conducted on an NH3/CO2 cascade refrigeration system to optimize the operating parameters. An NH3/CO2 cascade refrigeration system test platform was built, and the experimental data at some working points were obtained. Comparative and error analyses were performed using the thermodynamic analysis results of empirical formulas for different compressor isentropic efficiency. The formula that is consistent with the experimental data was used to analyze the maximum coefficient of performance (COPmax), the optimal intermediate temperature (TLC,opt), and the optimal inter-stage capacity ratio (nV,opt) of the system under variable temperature conditions. The temperature conditions included the evaporating temperature (Te), condensing temperature (Tc), cascade heat exchange temperature difference, and subcooling and superheating in the high-and low-temperature loops. The results show that Te has the greatest influence on COPmax and TLC,opt. When Te increased by 5 °C, the COPmax of the system increased by approximately 18.5%, and the TLC,opt increased by approximately 2.23 °C. Tc had the greatest influence on nV,opt, and the low-temperature loop superheat had the least influence on nV,opt. For every 5 °C increase in Tc, the nV,opt increased by approximately 6.34%. A correlation fitting formula between the performance parameters and working conditions is proposed, which can provide a reference for the optimization of operations in a cascade refrigeration system.

Zhang Shengshi, Zhao Yanjie, Li Zhaoning

In practical applications, pipeline transportation is usually used to transport ice slurry to the area to be cooled for heat exchange. Therefore, it is important to study the flow characteristics of ice slurries. This article is based on an ice slurry preparation and experiment in a flow characteristics test platform. The ice slurry was prepared with a 5% mass fraction urea solution, and the size and distribution of ice particles in the ice slurry were visually observed. The kinematic viscosity of the ice slurry was measured, and the relationships between the pipe diameter, ice packing factor (IPF), flow pressure drop, pipe frictional resistance coefficient, and Reynolds number (Re) were analyzed. The ratio of the experimental value (λ) of the friction resistance coefficient of the ice slurry in horizontal stainless-steel pipes with different pipe diameters to the theoretical value (λ0) was calculated by taking the ice slurry as a Newtonian fluid, and the relationship between the ratio and IPF and Re was analyzed. It was found that λ/λ0 increases with the increase in IPF and decreases with the increase in Re. That is, the ice slurry is closer to a Newtonian fluid under a high Reynolds number, whereas the deviation in ice slurry with high ice packing factor from a Newtonian fluid is larger. A power-law model was used to analyze the flow characteristics of the ice slurry. It was found that the flow characteristic index n' decreased with the increase in IPF. In a pipe with a diameter of 6.0 mm, n' gradually decreases from 1.006 under IPF = 6% to 0.611 under IPF = 26%; however, the consistency coefficient K' is positively correlated with the IPF. In a pipe with a diameter of 8.0 mm, K' increases from 0.015 under IPF = 6% to 0.274 under IPF = 26%. When the IPF is within the range of 5%–30%, n' decreases slightly with the increase in pipe diameter, whereas K' increases gradually with it. To better describe the complex flow characteristics of non-Newtonian fluids, a modified Reynolds number was introduced to quantitatively analyze the slurry. By exploring the relationship between the modified Reynolds number and Fanning friction coefficient, it was found that with the increase in the IPF, the range of transition Reynolds numbers in 4 mm, 6 mm, and 8 mm tubes were 2 500–3200, 1 600–2 300, and 1 500–1 900, respectively.

Liu Yusheng, Li Wanyong, Shi Junye et al.

The condenser is one of the important components in an automotive air conditioning system. A traditional condenser is designed on the premise that the inlet air volume is evenly distributed. However, in reality, condenser inlet air is not uniform due to the presence of the anti-collision beam, which makes the thermal performance rapidly decline. In this paper, a new design of condenser layout was studied. The middle region is the superheating area, the upper and middle lower parts are the two-phase area, and the lower part is the super cooling area, which expresses the maximum heat dissipation potential of the condenser. Experimental analyses demonstrated that the heat exchange capacity, COP, system flowrate, and other comprehensive performance indices of the condenser with new process layout were 10%–15% higher than a traditional condenser. Furthermore, this enhancement in performance was greater in the presence of a car anti-collision beam. The cooling effect of the condenser was 4 °C higher than that of the original construction, which greatly reduced the influence of the uneven distribution of air velocity caused by the car’s anti-collision beam on system performance.

马伟斌, 梅建滨, 李戬洪 et al.

吕树申, 王世平

An experimental study has been conducted to reveal the physical process of gas hydrate formatting and melting in the cool storage tank using Freon 134a as a cool storage medium.The effects of n-butanol on the formation of gas hydrate are investigated.The results show R134a is feasible for gas hydrate cool storage engineering application.

严雷, 刘斌, 邹同华

The matching of the catcher and the vacuum pump determines the pre-cooling quality during vacuum cooling processes.With the assumption of the water vapor resulted from the vacuum cooling to be ideal gas,the energy consumptions of the catcher and the vacuum pump needed in cooling were calculated,respectively.The total energy consumed of the system was also calculated.The vacuum pre-cooling experiment for the cabbage at the 60% package rate was performed.The results show that under a known working condition,the energy consumption per second is only dependent on the sucking rate of the vacuum pump,and the total consumed energy is dependent on the type of fruits or vegetables needed to be cooled and has no relations with the system parameters.