Chunxin Yang

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.

Evaluation of the wavelet transform method for machined surface topography I: methodology validation

In this paper, a new method based on wavelet transform is proposed as a means for studying the fractal characteristics of rough surfaces. Through estimation of normal mathematical curves with known fractal dimensions, generated by the Weierstrass-Mandelbrot function, Majumdar-Bhushan function, Fractal Brownian motion (including three methods: the Midpoint FBm, the Additions FBm, the Interpolated FBm) and Interposed method (Kiesswetter curve), it is validated that the wavelet transform method can accurately calculate the fractal dimension. These fractal functions have been used to simulate some surface profiles. The results indicate that the Wavelet transform method is the most precise in its calculation of the fractal dimensions of the curves. It obtains more accurate results than seven other methods, named the Box counting method, the Yardstick method, the Co-variation method, the Structure function method, the Variation method, the Power Spectrum method and the Rescaled range analysis method. Precisely calculating the fractal dimensions of the curves is the first step in characterising machined surface topography. In addition, this paper aims to further develop the evaluation procedure for the fractal characteristics of machined surface topography.

Evaluation of the wavelet transform method for machined surface topography 2: fractal characteristic analysis

Abstract The wavelet transform method is a new method for characterising the roughness profiles of machined surfaces. Based on the Weierstrass–Mandelbrot function and the Majumdar–Bhushan function with known fractal dimension, the wavelet transform method accurately calculates the fractal dimension. The fractal characteristics of test surfaces for pool boiling experiments are evaluated by the Wavelet transform method. Using general machining methods, these surfaces were prepared by rolling, hand and machine polishing copper and stainless steel. The results indicate that the wavelet transform method is more effective than seven other methods for calculating the fractal dimensions of roughness profiles. The topography of the surfaces is shown to be anisotropic. Although this method is developed to describe the test surfaces for boiling experiments, it may be more generally applicable in the field of tribology.

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.

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.

Research on the Environmental Control System of the Equipment Bay for A High Altitude Long Endurance Solar-powered UAV

Substantial efforts have been focused on developing high altitude long endurance solar-powered unmanned aerial vehicles (LESP UAVs). With increasingly integrated and compact electronic equipment, the demand for effective heat dissipation becomes more critical but challenging. Aiming at LESP UAV’s equipment bay with the cooling capacity demand of 5kW, after the availability of heat sink is analysed, a single-stage and two-stage vapor compression refrigeration cycle based on radiation cooling are proposed. By using thermodynamic analysis and quality estimation, the corresponding quantitative relationship is obtained, and the variation between cooling capacity and power consumption/system weight are calculated. Combined with the engineering application, the suitable conditions of two schemes are given. The results can provide theoretical and engineering guidance for the research in this field, and have good engineering application value.

Analysis on Air-Side Characteristics of the Condenser at Low Ambient Pressure Conditions

The vapor cycle refrigeration system is used in aircraft cooling. The changes of the ambient air properties will affect the condenser performance of the vapor cycle refrigeration system. The air-side characteristics between the flow and pressure drop at low pressure conditions must be taken into account, especially when it is used in the aircraft. Through the three tests of the condenser fan, the condenser, and the condenser module, the common property model of the air-side flow and pressure head/drop for each part is established and experimentally verified. The results show that the flow and pressure head or drop relationship of each unit under different pressures of 101.3kPa, 79.5kPa, 61.7kPa, and 47.2kPa is simply deduced by means of the ground test. From this study, the operation points of the condenser module under low ambient pressure conditions can be derived as well.Copyright