A heat exchanger that arranges flat tubes horizontally has a vertical header that distributes the refrigerant to each tube. When the heat exchanger works as an evaporator, differences in flow conditions at each branch...A heat exchanger that arranges flat tubes horizontally has a vertical header that distributes the refrigerant to each tube. When the heat exchanger works as an evaporator, differences in flow conditions at each branch, such as the ratio and distribution of vapor and liquid, due to the differences in densities and momentums of vapor and liquid in the two-phase flow make equal distribution difficult. This paper describes the distribution characteristics of a four-branch header that has a rectangular cross-section without the internal protrusion of flat tubes in the case of the inflow of the refrigerant R32 from the bottom of the header by using an equipment that can estimate the distribution ratio of the liquid and vapor phase to each branch. This paper also discusses the distribution characteristics on the basis of the flow visualization in the header. The flow visualization shows that a liquid level that contains vapor phase exists in the header and affects the distribution greatly.展开更多
We present a low cost radio-frequency （RF） generator suitable for experiments with cold atoms. The RF source achieves a sub-hertz frequency with tunable resolution from 0 MHz to 400 MHz and a maximum output power of...We present a low cost radio-frequency （RF） generator suitable for experiments with cold atoms. The RF source achieves a sub-hertz frequency with tunable resolution from 0 MHz to 400 MHz and a maximum output power of 33 dBm. Based on a direct digital synthesizer （DDS） chip, we implement a ramping capability for frequency, ampli- tude and phase. The system can also operate as an arbitrary waveform generator. By measuring the stability in a dura- tion of 600 s, we fred the presented device performs comparably as Agilent33522A in terms of short-term stability. Due to its excellent performance, the RF generator has been already applied to cold atom trapping experiments.展开更多
文摘A heat exchanger that arranges flat tubes horizontally has a vertical header that distributes the refrigerant to each tube. When the heat exchanger works as an evaporator, differences in flow conditions at each branch, such as the ratio and distribution of vapor and liquid, due to the differences in densities and momentums of vapor and liquid in the two-phase flow make equal distribution difficult. This paper describes the distribution characteristics of a four-branch header that has a rectangular cross-section without the internal protrusion of flat tubes in the case of the inflow of the refrigerant R32 from the bottom of the header by using an equipment that can estimate the distribution ratio of the liquid and vapor phase to each branch. This paper also discusses the distribution characteristics on the basis of the flow visualization in the header. The flow visualization shows that a liquid level that contains vapor phase exists in the header and affects the distribution greatly.
基金This work has been supported by the National Natural Science Foundation of China(No.51275523)the Specialized Research Fund for the Doctoral Program of Higher Education of China(No.20134307110009)
文摘We present a low cost radio-frequency （RF） generator suitable for experiments with cold atoms. The RF source achieves a sub-hertz frequency with tunable resolution from 0 MHz to 400 MHz and a maximum output power of 33 dBm. Based on a direct digital synthesizer （DDS） chip, we implement a ramping capability for frequency, ampli- tude and phase. The system can also operate as an arbitrary waveform generator. By measuring the stability in a dura- tion of 600 s, we fred the presented device performs comparably as Agilent33522A in terms of short-term stability. Due to its excellent performance, the RF generator has been already applied to cold atom trapping experiments.
