Xiulan Huai

Effect of heat treatment on the tribological behavior of 2D carbon/carbon composites

The effect of high temperature heat treatment on the tribological behavior of carbon/carbon (C/C) composites has been investigated. C/C composite preforms were made from 1K PAN plain carbon cloth, and densified using rapid directional diffusion (RDD) CVI processes. Four specimens treated at 1800, 1800+2000, 2000, and 2300 degreesC, respectively, were prepared. A ring-on-ring specimen configuration was used to simulate aircraft brakes. The brake initial angular velocity ranged from 1800 to 7500 rpm (6.2-26.0 m s(-1) average linear sliding velocity). The specific pressure and moment of inertia were 392-784 kPa and 0.25-0.31 kg m(2), respectively (1.9-42.3 MJ m(-2) kinetic energy loading per unit friction surface area). The results showed that the stability of the brake moment-time curves increased with increasing heat treatment temperature (HTT) for the four composites, and those treated at 2300 degreesC possessed the lowest initial brake moment peak ratio values (from 1.1 to 1.3). The high degree of graphitization and low shear forces of the matrix carbon resulting from the high HTT could allow friction films to develop and reduce those values under the present brake conditions. The friction coefficients of four RDD CVI C/C composites decreased with an increase in specific pressure. The resulting changes in the friction coefficient of the four composites due to the specific pressure changes have basically nothing to do with the interface temperature under those conditions. According to the practical brake conditions, the friction properties of RDD CVD C/C composites could be improved by regulating the structure of the brake discs, changing the specific pressure exerted on the discs and the heat treatment. The linear wear rates of the four materials increased with increasing HTT. The composites treated at 2000 degreesC had both high enough friction coefficients and the lower linear wear rates. The different heat treatment methods at 2000 degreesC had no obvious effect on the friction and wear properties of RDD CVI C/C composites

Multi-Phase Flow and Drying Characteristics in a Semi-circular Impinging Stream Dryer

A physical model was proposed to describe granular material drying in a semi-circular impinging stream dryer, and the multi-phase flow characteristics as well as the heat and mass transfer were numerically investigated. Specially, the influence of various factors (inlet air temperature, mass flow-rate ratio, initial moisture content etc.) on drying process was inspected. The results indicate that constant drying rate period does not exist in a semi-circular impinging stream dryer. Appropriate curvature radius, flow-rate ratio, air velocity and higher inlet air temperature should be chosen for improving the drying performance, and decreasing the energy consumption and operation cost. The numerical predictions were compared with the available experimental results, and they are in quite good agreement with each other.

Heat transfer characteristics of supercritical CO2 flow in small-channeled structures

Carbon dioxide (CO2) has been proposed as a “natural refrigerant” for various kinds of refrigeration systems and has attracted increased attention from researchers and engineers all over the world. The heat transfer and flow characteristics of supercritical CO2 in a multi-port extruded aluminum test section, which had ten circular channels with inner diameter of 1.31 mm, were investigated experimentally. The CO2 was cooled with chilled copper blocks attached at the both sides of the test section. Local heat fluxes and temperatures at the outer surface of the test section were measured using 12 heat flux sensors and 24 K-type thermocouples, respectively. The mean temperatures of CO2 at the inlet and outlet of the test section were also measured. The higher heat transfer rates were achieved in the near-critical region, and the system pressure, mass velocity, and temperature of CO2 all were observed having significant effects on the heat transfer. An empirical correlation was then developed for forced convective heat transfer of supercritical CO2 in horizontal micro/mini channels.

Analysis of thermal effects in an orthotropic laser medium

The temperature, thermal stress, thermal strain, and optical path difference (OPD) in an orthotropic laser medium under Gaussian, top-hat, and uniform pumping schemes are solved both analytically and numerically. The results indicate that, provided the same total heat loading, the thermal effects under the top-hat pumping scheme are lower than under the Gaussian pumping scheme, whereas the thermal effects under uniform pumping are the least significant of all; in the absence of external forces, the orthotropic thermal properties have more significant effects on the thermal strain than on the thermal stress. The theoretical OPD agrees well with published experimental data and shows evident orthotropy.

Thermosolutocapillary Convection in an Open Rectangular Cavity With Dynamic Free Surface

Thermosolutocapillary convection within a rectangular cavity with dynamic free surface is numerically investigated in the absence of gravitational effects. Both the temperature and solute concentration gradients are applied horizontally. The free surface deformation is captured by the level set method. Two cases of the ratio of thermal to solutal Marangoni number R-sigma < -1 and R-sigma = -1 are considered. For R-sigma < -1, the free surface bulges out near the left wall and bulges in near the right wall; with the increase of Marangoni number, the free surface deformation decreases and with the increase of capillary number and aspect ratio, it increases. For R-sigma = -1, the free surface bulges out near the left and right walls and bulges in at the central zone; with the increase of Marangoni number, the free surface deformation mode is changed and with the increase of capillary number and aspect ratio, the free surface deformation increases.

Cavitation Bubble Collapse Near a Heated Wall and Its Effect on the Heat Transfer

In the present work, a numerical investigation on the mechanism of heat transfer enhancement by a cavitation bubble collapsing near a heated wall has been presented. The Navier-Stokes equations and volume of fluid (VOF) model are employed to predict the flow state and capture the liquid-gas interface. The model was validated by comparing with the experimental data. The results show that the microjet violently impinges on the heated wall after the bubble collapses completely. In the meantime, the thickness of the thermal boundary layer and the wall temperature decrease significantly within the active scope of the microjet. The fresh low-temperature liquid and the impingement brought by the microjet should be responsible for the heat transfer reinforcement between the heated wall and the liquid. In addition, it is found that the impingement width of the microjet on the heated wall always keeps 20% of the bubble diameter. And, the enhancement degree of heat transfer significantly depends on such factors as stand-off distance, saturated vapor pressure, and initial bubble radius.

Thermal stress analysis of a Nd:YVO4 laser medium end pumped by a Gaussian beam

The heat conduction and the thermal stress problems in an orthotropic Nd:YVO4 laser medium end pumped by a Gaussian beam are modeled and analytically solved. The three-dimensional temperature expression is derived via an integral transform method, and the thermal stress field is deduced by Airy’s thermal stress function method, utilizing said three-dimensional temperature expression, based on the plain-strain assumption. The results indicate that the orthotropic thermal properties have significant influence on the thermal strain pattern, but have less effect on the thermal stress profile in the absence of external forces.

Influence of longitudinal rise of coolant temperature on the thermal strain in a cylindrical laser rod

The thermal strain in a laser rod with a longitudinal temperature increase is modeled and analytically derived through the method of thermoelastic displacement potential and the method of Love displacement function. The analytical results show that in the absence of external forces, the longitudinal rise of fluid temperature has an unnoticeable effect on the thermal stress profile in the laser rod. However, the thermal strain field caused by the temperature distribution under the traction free boundary condition has an evident variation in the longitudinal direction, which will considerably affect the laser transmission characteristics and the beam quality.

A novel device for hazardous substances degradation based on double-cavitating-jets impingement: Parameters optimization and efficiency assessment

Hydrodynamic cavitation is an effective advanced oxidation process. But sometimes it cannot obtain satisfactory treatment efficiency by using hydrodynamic cavitation individually, so it is necessary to introduce intensive methods. Based on double-cavitating-jets impingement, this paper presents a novel device that has advantages of strong heat and mass transfer and efficient chemical reactions. Based on the device, a series of experimental investigations on degradation of a basic dye, i.e. Rhodamine B were carried out. Significant Rhodamine B removal from aqueous solution was observed during 2h treatment and the degradation reaction conformed to pseudo-first-order kinetics. The synergetic effects between double-cavitating-jets impingement and Fenton chemistry on simultaneous degradation of Rhodamine B were confirmed. Both single-variable experiments and orthogonal experiments were carried out to study the effects of initial hydrogen peroxide, ferrous sulfate and Rhodamine B concentrations and the optimum conditions were found out. Effects of jet inlet pressure in the range of 6-12MPa and solution pH value in the range of 2-8 were also investigated. The cavitation yield was evaluated to assess the energy efficiency. The present treatment scheme showed advantages in terms of reducing the demand of hydrogen peroxide concentration and enhancing the treatment efficiency in large scale operation.

Effects of Chemical Heat Sink Generated by an Oxalic Acid Cooling Stream on Film-Cooling Effectiveness

Based on the theory of heat transfer enhancement, a novel film-cooling method for turbine blades that involves a chemical heat sink generated by the use of oxalic acid as a cooling stream has been proposed. Experiments were conducted on a flat plate with a row of 30° angled holes, and the conventional method using air as the cooling stream was examined for comparison. Overall, the results of this study demonstrate that the proposed cooling method involving an oxalic acid-induced chemical heat sink is more effective than the conventional method.