Experimental investigation on startup and thermal performance of a high temperature special-shaped heat pipe coupling the flat plate heat pipe and cylindrical heat pipes

A high temperature special-shaped heat pipe (HTSSHP) coupling the flat plate heat pipe (FHP) and cylindrical heat pipes (CHPs), respectively as the thermal receiver and heat transfer unit, is proposed as the key component in a solar thermochemical reactor. The startup characteristics, isothermal performance, and thermal resistance variation of HTSSHP were experimentally investigated and analyzed. The results show that heat transfer limits are not encountered in tests and HTSSHP can startup effectively. The continuum flow transition temperature of sodium vapor in FHP and CHPs are 426.0 °C and 442.3 °C respectively and the startup behavior of HTSSHP differs from the flat-front phenomenon in a traditional high temperature cylindrical heat pipe with a constant transition temperature each part. However, the temperature difference response in startup can be employed to indentify the sodium melting and the flow regime transition in FHP and CHPs, validating its startup law. HTSSHP possesses the potential to inhibit the hotspots on heat absorber surface and meanwhile improve the temperature distribution in reaction chamber. The overall thermal resistance in HTSSHP reduces with increasing operating temperature, ranging from 0.12 to 0.19 °C/W that is in the same order of magnitude in a typical heat pipe. Furthermore, the various heat inputs and cooling rates have considerable effect on the thermal resistance in cooling side. This work is helpful for establishing the preliminary understanding of the operating characteristics on HTSSHP and provides some suggests for its normal operation and structure optimization.