Yuting Wu

Drop distributions and numerical simulation of dropwise condensation heat transfer

Abstract The random fractal model is firstly put forward to simulate the drop size and spatial distribution in dropwise condensation. The boundary conditions of heat conduction through the condensing wall surface are then established using the heat transfer model through a single drop proposed by J.W. Rose. A numerical method towards the directed simulation of dropwise condensation heat transfer is presented. The model considered the effect of the nonuniformity of surface heat transfer thermal conductivity of the condenser material on dropwise condensation. Numerical computations are conducted for dropwise condensation heat transfer of water on the copper wall on a very wide range of pressure. The numerical simulation results agreed well with the bulk of existing experimental data and the precision is higher than the model proposed by Rose.

Synergistic Effect of Nanophotocatalysis and Nonthermal Plasma on the Removal of Indoor HCHO

Photocatalysis is an effective method of air purification at the condition of a higher pollutant concentration. However, its wide application in indoor air cleaning is limited due to the low level of indoor air contaminants. Immobilizing the nanosized TiO2 particles on the surface of activated carbon filter (TiO2/AC film) could increase the photocatalytic reaction rate as a local high pollutant concentration can be formed on the surface of TiO2 by the adsorption of AC. However, the pollutant removal still decreased quickly with the increase in flow velocity, which results in a decrease in air treatment capacity. In order to improve the air treatment capacity by the photocatalytic oxidation (PCO) method, this paper used formaldehyde (HCHO) as a contaminant to study the effect of combination of PCO with nonthermal plasma technology (NTP) on the removal of HCHO. The experimental results show that HCHO removal is more effective with line-to-plate electrode discharge reactor; the HCHO removal and the reaction rate can be enhanced and the amount of air that needs to be cleaned can be improved. Meanwhile, the results show that there is the synergistic effect on the indoor air purification by the combination of PCO with NTP.

Optimal analysis of a space solar dynamic power system

The major purpose of the present study is the theoretical modeling, numerical simulation and optimal analysis of a space solar dynamic power system. Using the method of system analysis, a mathematical and physical model is developed to describe the process of energy transfer and conversion in a space solar dynamic power system. As a new assessing criterion for total launch mass, it is proposed to combine the system mass and aerodynamic drag area into a unified criterion. The effects of the configurations and operating parameters on the system performance are analyzed and the optimal scheme of a space solar dynamic power system is obtained by numerical simulation.

Experimental Study on the Influence of Rotational Speed on the Performance of a Single-screw Expander with a 175 mm Screw Diameter

A single-screw expander has been designed and manufactured independently. Based on this prototype, testing system has been built and performance experiment has been made. In this article, compressed air was used as working fluid and performance test for the prototype was finished at conditions including different rotational speed and different inlet pressure. From the experimental data, it is shown that when inlet pressure less than 0.8MPa the output power increases with the increase of rotational speed because of not enough expansion; when inlet pressure more than 0.8MPa, the every biggest output power is appeared in the condition of rotational speed 2600 rpm. The test results also show that the total efficiency is influenced by rotational speed obviously, and the highest total efficiency of this machine is 69.64% in the condition of 3000 rpm and 15 bar.

A THEORETICAL STUDY OF THE EFFECT OF SURFACE THERMAL CONDUCTIVITY ON HEAT TRANSFER COEFFICIENT IN DROPWISE CONDENSATION

Whether there is a significant effect of surface thermal conductivity on heat transfer coefficient in dropwise condensation is a problem that has not yet been settled. It is impossible to settle the discrepancy only by experimental study. Numerical simulation of dropwise condensation heat transfer of steam on copper, stainless steel, zinc, and carbon steel surfaces is performed using a model of dropwise condensation heat transfer proposed by the authors. The results indicate that the heat transfer coefficient is dependent upon the surface thermal conductivity, and the heat transfer coefficient of dropwise condensation decreases with the surface thermal conductivity.Whether there is a significant effect of surface thermal conductivity on heat transfer coefficient in dropwise condensation is a problem that has not yet been settled. It is impossible to settle the discrepancy only by experimental study. Numerical simulation of dropwise condensation heat transfer of steam on copper, stainless steel, zinc, and carbon steel surfaces is performed using a model of dropwise condensation heat transfer proposed by the authors. The results indicate that the heat transfer coefficient is dependent upon the surface thermal conductivity, and the heat transfer coefficient of dropwise condensation decreases with the surface thermal conductivity.

Experimental Study on the Performance of the Single Screw Expander Prototype by Optimizing Configuration

Improving thermodynamic efficiency of prime movers is the key issue for efficient utilization of low temperature heat resources. Because of many good characteristics, the single screw expander is perhaps a good choice. Precisions in manufacture and assembly are very important factors to the performance of single screw expanders. In this paper, experimental results of the first single screw expander prototype made by ourselves was analyzed and reported for the first time. From the experimental data, the power output of the prototype was about 5kW, but the gas consumption rate was above 105kg/kWh, so the shaft efficiency was only 34% or so. The results indicated that internal leakage was very serious. Hence, the second single screw expander prototype was manufactured. The gap between gate rotor and shell of the new prototype was reduced. The gap is about 0.02mm. From the new experimental data, the mass flow rate was significantly decreased. However, the power output was only 1.4kW. So, the gas consumption rate was even more than the first prototype. The reason of bad experimental results may arise from the too large friction between screw, gate rotor and shell in the case of small gap. So, we manufactured the third single screw expander prototype, which is 0.04mm in gap. From the experimental data, the maximum power output was about 4.5kW, but the mass flow rate was decreased sharply. So the gas consumption rate was about 60kg/kWh, and the shaft efficiency was about 60%. The result indicated that the performance of the single screw expander was remarkable improved by optimizing its configuration.Copyright

Efficiency Analysis on Low Temperature Energy Conversion System Based on Organic Rankine Cycle

The amount of low temperature heat resources is very huge, efficient utilization that energy is very important issue for improving energy efficiency, saving energy and protecting environment. Due to the small available energy of low temperature heat source, how to improve thermodynamic efficiency is the key problem. In this paper, the thermodynamic model of low temperature thermal power conversion system based on organic Rankine cycle was described firstly. Turbine, single screw and piston expanders were briefly described. R123, R245fa and R134a were chose as working fluid because of quite different critical temperature. Based on this model, the influence of thermodynamic property of organic working fluid on the efficiency of low temperature thermal power conversion system was discussed. The calculating result showed that R123 is the best choice if no considering the impact of expander types and that R245fa is the best choice if considering the impact of expander. This conclusion indicated that it is very important to investigate the match relationship between working fluid and expander. Moreover, single screw expander was proved to be more suitable than turbine and piston expanders for low temperature heat power conversion system.

Research of Two Stage Single Screw Expander Organic Rankine Cycle System Scheme Based on the Waste Heat Recovery of Diesel Engine’s Exhaust Gas

According to the analysis of heat balance, about 1/3 of the fuel combustion heat is taken away into the ambience by exhaust gas of diesel engine. In this article, to improve the using level of the fuel’s combustion heat, a two stage single screw expander organic Rankine cycle (ORC) system has been used to recover the waste heat from exhaust gas of a certain turbine diesel engine. In this article, physical model of the recovery system was built at first, then the T-S curve was drawn, at last, REFPROP was used to calculate thermodynamics parameter in different state point of this system, and analyze the whole system’s thermodynamics character. By analyzing, the evaporation temperature of this system should be optimized to get the relatively evaporation press; by calculating, it could be seen that the middle heater in this system should be taken away to improve the economy of this scheme. This scheme should supply a direction for the exhaust heat recovery of diesel engine.

Experimental Study on Thermal Performance of Mixed Nitrate and Carbonate Salts

Although molten salts have been used in large scale in the experimental or commercial solar thermal power plants, systematic studies on thermal performence of mixed-salts are lacking. Four eutectic nitrate salts with low melting point were choosed by the analysis of the TG curve and DSC curves with nine binary mixtures of salts with different mass ratio, the specific heat-temperature curve were obtained after further analysis of the DSC curve and experimental correlations were fitted. Potassium carbonate, lithium carbonate, sodium carbonate are mixed in accordance with the different proportions, 36 kinds of mixed molten salt are obtained. The data of melting point, decomposition temperature, specific heat, latent heat and use temperature range were obtained by the analysis of the TGA and DSC curves of 36 salts.Copyright

Comparative study of impact of compressor speed on system performance subject to micro-gravity

Vapor compression heat pump has good prospects in future large-scale spacecraft thermal control technology. Compressor is a key component in vapor compression heat pump. It is essential to study its adaptability under micro-gravity. In this paper, an experimental system was built up to test the performance of inverted compressor subjected to micro-gravity. The performance of compressor upright and inverted was compared. The influence of compressor speed on the performance of vapor compression heat pump was investigated. The results showed that the inverted compressor could be considered gravity-independent because of normal operation. Compressor discharge temperature and power increased with compressor speed. However, the COP(Coefficient of Performance) was less than that of compressor upright. The maximum COP obtained with upright and inverted compressor were 8.4 and 7.6, respectively. The experiment subject to micro-gravity of vapor compressor heat pump provides an experimental basis and lays foundation for future space applications.