On thermal processes in the calibration and control of liquid calorimeters for measuring the energy of high-power microwave pulses

The heat transfer processes associated with the heating of the operating liquid and the calibration of the liquid calorimeters for measuring the energy of high-power microwave pulses are studied analytically and by numerical simulation. It is shown that convection significantly enhances heat transfer between the calibration and thermostabilization heaters and the operating liquid, hence, it helps to reduce the formation of bubbles in the volume of the operating liquid when controlling and calibrating the calorimeter. Heat transfer from tape heaters is noticeably more efficient than that from single wire ones. The results obtained are essential for the development of liquid calorimeters and the adjustment of their operating modes.

Effect of Microwave Pulses on the Morphology and Development of Spark-Ignited Flame Kernel

Microwave-assisted spark ignition (MAI) is a promising way to enhance the ignition performance of engines under lean conditions. To understand the effect of microwave-induced flow during MAI, the development and morphology of spark-ignited methane-air flame kernel under various microwave pulse parameters are experimentally studied. Experiments are conducted in a constant volume combustion chamber, and flame development is recorded through a high-speed shadowgraph method. Flame area and deformation index are adopted to evaluate the flame characteristic. Results show that increasing the microwave pulse energy from 0 to 150 mJ exhibits a threshold process for expanding the flame kernel area under 0.2 MPa ambient pressure. When the pulse energy is below the threshold of 90 mJ, the microwave enhancing efficiency is much lower than that beyond the threshold. Increasing microwave pulse repetition frequency (PRF) changes the flow on flame surface and raises the absorption efficiency for microwave energy, and thus helps to improve the MAI performance under higher pressures. Hence, 1 kHz pulses cause more obvious flame deformation than those with higher PRF pulses under 0.2 MPa, while this tendency is reversed as the ambient pressure increases to 0.6 MPa. Besides, microwave pulses of different repetition frequencies lead to different flame kernel morphology, implying the various regimes behind the interaction between a microwave and spark kernel.

Impact of The Structure on The Thermal Burnout Effect Induced by Microwave Pulses of PIN Limiter Diodes

Positive-intrinsic-negative (PIN) limiter are widely used to protect sensitive components from leakage power itself and adjacent high-power injection. Being the core of a PIN limiter, the PIN diode is possible to be burnt out by the external microwave pulses. Here, numerical simulations by our self-designed device-circuit joint simulator were carried out to study the influences of the I layer thickness and the anode diameter of the PIN diode on the maximum temperature variation curve of the PIN diode limiter. The damage threshold criterion in the numerical simulation was first studied by comparing experimental results with simulation results. Then, we determined the impact of the structure on the thermal burnout effect induced by microwave pulses of PIN limiter diodes.