Search

Due to the isotropy and high thermal conductivity of metal foam, a novel idea of isothermal chamber filled with graded copper foam (GCF) is developed to improve the isothermal characteristic of isothermal chamber. Firstly, an analytical effective thermal conductivity (ETC) prediction model of metal foam is proposed, with relative root-mean-square error (RRMSE) is 6.90%, which has higher ETC prediction accuracy than the semi-empirical models in the literatures. Secondly, the conventional porous random fiber bundle is replaced by the copper foam as the filler material to improve the isothermal characteristic, and this more accurate ETC prediction model of metal foam is used in the numerical simulation of the discharge process to determine the isothermal characteristic of isothermal chamber filled with homogeneous copper foam (HCF). Experiments are conducted to verify the results of simulation. Thirdly, the graded filling method is adopted to form an isothermal chamber filled with GCF, which enhance the heat transfer efficiency and further improve the isothermal characteristic. The results show that, compared with the conventional isothermal chamber filled with homogeneous porous random fiber bundle, the isothermal characteristic of the isothermal chamber filled with GCF is improved by 32.7%.

Two-phase closed thermosyphons (TPCTs) have been widely used in permafrost regions because of their good cooling effect. The variation in the inclination angle of TPCTs significantly affects the heat transfer process and temperature distribution inside TPCTs, which is still not fully understood. This study experimentally investigated the heat transfer performance of TPCTs with inclination angles varied from 0° to 90° with 10° intervals. In addition to the outer wall temperature and heat flux, the thermal regime of the working medium inside TPCT was simultaneously monitored based on the core-tube. Experimental results show that the temperature distributions inside TPCT along the axial direction vary regularly with inclination angle. The heat transfer rates of the evaporator and condenser sections change nonmonotonically and nonlinearly with the inclination angle. The influence of the inclination angle on the thermal resistance is greater in the evaporator and the adiabatic sections than in the condenser section. Consequently, a TPCT at 10° produces the best heat transfer performance, corresponding to the best isothermal characteristics, the maximum heat transfer rates of the evaporator and condenser sections, and the minimum total thermal resistance. This study provides a reference of monitoring method for further research on TPCT.

Isothermal chamber is filled with a certain density of high thermal conductivity porous material, which has a wide range of applications in the flow measurement of pneumatic field, and its isothermal characteristic is critical to its application results. In this paper, in order to improve the numerical calculation accuracy of the isothermal characteristics during discharge process, a discharge model of isothermal chamber with fractal effective thermal conductivity (ETC) for determining the isothermal characteristic is reported. Firstly, the stuffer in isothermal chamber is considered as porous random fiber bundle, and an ETC prediction model of anisotropic porous random fiber bundle in both vertical and horizontal directions is established by fractal theory. This model with two directions is in good agreement with the experimental results, and the relative root mean square errors (RRMSE) are 3.94% and 9.85%, respectively. Secondly, the discharge model with fractal ETCs is built, and the isothermal characteristics of isothermal chambers with three different porosities are determined by numerical simulation. Finally, experiments to determine the isothermal characteristic are carried out. The numerical simulation results are in good agreement with the experimental results, and the relative errors are less than 3%. It could be concluded that accurately determining the ETC of the stuffer in isothermal chamber can improve the numerical calculation accuracy of the isothermal characteristic. Moreover, compared with the experimental method, numerical method is energy-saving and timesaving.

The temperature drop during isothermal chamber discharging is obtained by the “stop method” generally. One of the most important things is to keep sealing during the whole process according to the test principle. As a result, the temperature changing is very difficult to be determined and the whole process is time-consuming. This study aims to find out a new method to forecast the temperature drop based on the convective heat transfer model for isothermal chamber during discharging. First, four kinds of isothermal chambers were discharged by four solenoid valves with known flow rate characteristics respectively, and the pressure curves and temperature curves during discharging were obtained by experiments. Second, based on the thermodynamic model and combined with pressure curves and temperature curves, the convection heat transfer empirical formulae were achieved according to the similarity principle, which reflect the influence of not only the discharging speed and the copper wire filling density but also the air compression ratio to heat transfer. Third, four different isothermal chambers were discharged by the two different solenoid valves, and pressure and temperature data were obtained by experiments. In addition, the temperature drop could be obtained by analyzing the pressure data combined with the thermodynamic model including the convective heat transfer empirical formulae. Compared the analyzed temperature drop with that obtained by using the “stop method”, it can be found that the difference is within the acceptable range, which shows that the convection heat transfer empirical formulae were reasonable for predicting the temperature drop for isothermal chambers during discharging. [ABSTRACT FROM AUTHOR]