Pneumatic temperature control of the isothermal region of the PCLHP

The pressure-controlled loop heat pipe (PCLHP) is a recent modification of the loop heat pipes, which was devised for precise temperature control rather than heat transportation. In the PCLHP, a cylindrical isothermal space is formed in a high-speed vapor flow region, and the vapor temperature flowing around the isothermal space is precisely controlled by pneumatically controlling gas pressure in the compensation chamber. This device is developed to realize an isothermal region of a finite size and to achieve a temperature control of the isothermal region at an unprecedented speed and precision, which is denoted as the pneumatic temperature control technique.

The pneumatic (formerly proposed as hydraulic) temperature control technique utilizes a thermo-hydraulic linkage between the saturated vapor temperature in the evaporator and the pressure in a physically separated but thermo-hydraulically linked two-phase reservoir (i.e., the compensation chamber) of the PCLHP. Thus, fast, precise and predictable temperature control of the isothermal region in any directions and scales is attainable by changing the pressure in the compensation chamber; instantaneous and stable control of the isothermal region temperature is attained with a stability of approximately 10 mK, and very fast temperature change without overshoots or undershoots is achieved under specified operating conditions. In addition, the controlled temperature is accurately predicted by the thermodynamic relation (i.e., the Clapeyron-Clausius approximation).

Comparison of heat spreading characteristics of the vapor chamber-type heat spreader with that of a copper plate
(left: vapor chamber-type heat spreader, right: copper Plate)

In this work package, a thin, flat heat spreader is devised for thermal control of ultra-high heat flux electronic devices (e.g., MMIC PA, IGBT, etc.). One of the main interests of this work package is on development of fit-to-purpose designs of heat spreaders for various applications based on passive two-phase heat transfer technologies. In addition, quantification of the heat spreading characteristics of the heat spreader, with accurate contact and non-contact thermometry techniques, is the most distinguished aspect of this work. Overall, systematic design of a heat spreader and reliable quantification of its heat spreading characteristics is the ultimate goal of this work package.

Flexible flat-evaporator loop heat pipe (FFELHP)

Recent progresses in modern electric systems such as electric vehicles require a highly efficient yet effective heat transport device to deliver a heat generated in the inverter or battery to a physically separated heat dissipating element. In this regard, passive two-phase heat transfer devices such as heat pipes, thermosyphons, and loop heat pipes, are right one to be used for thermal control of the electric systems.

Thermometry is the art of measuring temperature. Temperature measurements may span a large range, from ultra-cold temperatures of –273 °C up to temperatures of several thousands of °C . A precise and accurate knowledge of temperature is important in science, technology and industry where precision and pushed limits are targeted. In the related works, I aim to achieve the highest-level accuracy in thermometry, in particular, in the realization of the reference temperatures of the current international temperature scale (i.e., the fixed-point temperatures of the ITS-90 (International Temperature Scale of 1990). In addition, special thermometry techniques such as temperature measurements at high temperatures (e.g., near 1000 °C) and the high-speed resistance thermometry with low quadrature effect are of particular interest. Industry-wise applications of the thermometry is one of the the ultimate goals of the research.

The liquidus temperature is the temperature at which a molten sample begins to freeze (or a solidified sample finishes melting), and with a homogeneous solute composition and at a particular pressure, the liquidus temperature of a sample is uniquely determined. The liquidus temperature can be determined by melting the fixed-point sample with discrete heat pulses and adiabatically measuring the melting temperatures at specific melted fractions between the heat pulses (i.e., the heat pulse-based melting). Based on the pneumatic temperature control technique, the heat pulse-based melting of a high purity (nominally 99.9999%) tin was carried out using square wave-type temperature steps generated by the pressure-controlled LHP with periodic pressure steps. The melting was carried out by applying the controlled heat pulses to the sample, and the melting temperatures at specific melted fractions were adiabatically measured between the heat pulses; the liquidus temperature was finally determined by extrapolating the melting temperatures to the melted fraction of unity (i.e. the end of the melt).