Fracture toughness is used to characterize rock resistance to fracturing and it is important in theoretical research and engineering applications. Mode-I fracture toughness can be predicted on the basis of an empirical relationship between fracture toughness (KIC) and tensile strength (σt). In underground engineering, rocks are often subjected to different temperatures. Therefore, this paper explores the effect of temperature on the relationship between mode-I fracture toughness and tensile strength. The results show that the change trends in the KIC and σt values of rocks at temperatures from 20 °C to 600 °C are broadly consistent with each other. For rocks heat-treated to the same temperature, the KIC of the rock increases with an increase in σt. This positive correlation between KIC and σt is different in rocks heat-treated to different temperatures. Critical crack propagation radius (rIC) is an important factor in the relationship between KIC and σt and is related to the type of rock and the conditions under which it is tested. For the same rock, rIC is quite different after it has been exposed to different temperatures. The positive correlation between KIC and σt results from a similarity in the fracture morphology and properties of failure when rock is destroyed in fracture and tensile tests.

Due to its distinct capability to improve the efficiency of shale gas production, supercritical carbon dioxide (SC-CO 2 ) fracturing has attracted increased attention in recent years. Heat transfer occurs in the transportation and fracture processes. To better predict and understand the heat transfer of SC-CO 2 near the critical region, numerical simulations focusing on a vertical flow pipe were performed. Various turbulence models and turbulent Prandtl numbers (Pr t ) were evaluated to capture the heat transfer deterioration (HTD). The simulations show that the turbulent Prandtl number model (TWL model) combined with the Shear Stress Transport (SST) k-ω turbulence model accurately predicts the HTD in the critical region. It was found that Pr t has a strong effect on the heat transfer prediction. The HTD occurred under larger heat flux density conditions, and an acceleration process was observed. Gravity also affects the HTD through the linkage of buoyancy, and HTD did not occur under zero-gravity conditions.

Abstract:The current research on confined submerged waterjets mainly focuses on the flow field of the impinging jet and wall jet. The double-sided wall vertically confined waterjet, which is widely used in many fields such as mining, cleaning and surface strengthening, has rarely been studied so far. In order to explore the influence of a double-sided wall confined width on the velocity field of submerged waterjet, an experiment was conducted with the application of 2D particle image velocimetry (PIV) technology. The distribution of mean velocity and turbulent velocity in both horizontal and vertical planes was used to characterize the flow field under various confined widths. The results show that the vertical confinement has an obvious effect on the decay rate of the mean centerline velocity. When the confined width changes from 15 to 5, the velocity is reduced by 20%. In addition, with the decrease of the confined width, the jet has a tendency to spread horizontally. The vertically confined region induces a space hysteresis effect which changes the location of the transition region moving downstream. There are local negative pressure zones separating the fluid and the wall. This study of a double-walled confined jet provides some valuable information with respect to its mechanism and industrial application.

To make a better application of the self-excited oscillation jet, a compound organ–Helmholtz oscillation nozzle (OH nozzle) was designed to generate better pulse effects in the present study. The effects of geometric parameters on pressure characteristics of self-excited oscillation jets were investigated experimentally. The geometric parameters of OH nozzles were determined based on the design principle of the organ-pipe and Helmholtz nozzles. Various types of OH nozzles were tested to obtain the relationship between oscillation pressure and geometric parameters. Experimental results showed that some structural parameters are sensitive to the pump pressure, while others are not. The optimum geometric optimum parameters were obtained based on experimental results. The peak pressure and pressure pulsation amplitude are closely related to the target distance. The peak pressure decreases sharply with target distance at first and decreases with a further increase of the target distance. The peak pressure becomes stable beyond a certain range of target distance. With the increase of the target distance, the pressure pulsation amplitude increases at first and then decreases. The optimum target distance was determined based on experimental results to achieve better pulsation performance and has a value of 20 mm in the present study.