The Development Law of the Freezing Temperature Field of a Calcareous Clay Layer

The problem of “difficult” freezing of the calcareous clay layer fractures in freezing pipes has been investigated. Based on the engineering background of the deep calcareous clay in the Yangcun Mine, model tests were carried out in order to conduct in-depth research on the development law of the freezing temperature field of this clay layer. The test results have shown that the calcareous clay has a freezing point of −1.3°C under the action of both the water and the soil’s chemistry and the supercooling temperature can be as low as −3.8°C because of its complex mineral composition causing poor thermal conductivity. This means that the calcareous clay will freeze slowly than the other layers of the soil. The time taking for the temperature fields to intersect is 2.5 h, which is equivalent to 127.6 days in the actual engineering. In the three sections, each temperature measurement point in the temperature field had an irregular saddle shape in the temperature space at the same time, and the ratio of the time between the formation and total melting of the frozen wall was 1 : 1.91. The development speed of the thickness of the frozen wall from 5 h to 16 h was 17.9 mm/h, and the development speed from 16 h to 70 h was 1.96 mm/h; corresponding to the actual development speed of the thickness of the frozen wall which were 0.0123 m/d and 0.0014 m/d, respectively. These speeds were significantly slower than the development speed of the thickness of the freezing wall of the general sandy clay layer, which were 0.0515 m/d in the early stage and 0.02 m/d in the later stage. The thin thickness and low strength of the frozen wall of the calcareous clay layer cause the fracture of the frozen pipes, which should be paid attention to in actual engineering construction.

Study on Temperature Measurement Point Optimization and Thermal Error Modeling of NC Machine Tools

Background: In precision machining, thermal error is the main source of machine tool error. And thermal error compensation is an effective method to reduce thermal error. Objective: In order to improve the prediction accuracy and computational efficiency of thermal error model, a new optimization method used for the selection of temperature measurement point is proposed. Method: This method is based on stepwise regression. According to the results of partial-F statistic, new variable is selected one by one, unapparent variables are deleted, and optimization selection of temperature measurement point is fulfilled, thermal error model of the NC machine tool is presented. Result: The new modeling method was used on NC machine tool, which reduced the temperature point number from 24 to 5. Moreover, model residual was less than 5µm after compensation. Conclusion: The result shows that the new thermal error model has higher prediction accuracy and less temperature variables.

Evaluation of a Coaxial Borehole Heat Exchanger Prototype

Different borehole heat exchanger designs have been discussed for many years. However, the U-pipe design has dominated the market, and the introduction of new designs has been practically lacking. The interest for innovation within this field is rapidly increasing and other designs are being introduced on the market. This paper presents a general state of the art summary of the borehole heat exchanger research in the last years. A first study of a prototype coaxial borehole heat exchanger consisting of one central pipe and five external channels is also presented. The particular geometry of the heat exchanger is analyzed thermally in 2-D with a FEM software. An experimental evaluation consisting of two in situ thermal response tests and measurements of the pressure drop at different flow rates is also presented. The latter tests are carried out at two different flow directions with an extra temperature measurement point at the borehole bottom that shows the different heat flow distribution along the heat exchanger for the two flow cases. The borehole thermal resistance of the coaxial design is calculated both based on experimental data and theoretically.