Theoretical Investigation of Buoyancy-Driven Two-Phase Heat Transfer Enhancement Mechanisms in Wickless Loop Thermosyphon Heat Pipes for Solar Thermal Applications in Nigeria

The theoretical mechanisms governing buoyancy-driven two-phase heat transfer enhancement in wickless loop thermosyphons operating with working fluids at conditions relevant to solar thermal energy collection in tropical Nigeria represent an incompletely resolved area of heat transfer theory, with existing analytical models exhibiting systematic prediction errors that limit confident design of high-performance solar thermal systems for the Nigerian climate. This research conducts a theoretical investigation of the coupled fluid dynamic and heat transfer mechanisms in closed two-phase loop thermosyphons using analytical modelling, scale analysis, and high-resolution computational fluid dynamics to develop improved predictive theories for evaporator pool boiling, vapour-liquid counter-current flow in the riser, and condenser film condensation under the specific operating pressure and temperature ranges relevant to solar water heating in Nigeria. A novel theoretical framework is developed that incorporates the effects of dissolved non-condensable gases, which accumulate from working fluid outgassing under tropical storage conditions, on vapour-liquid interfacial dynamics and heat transfer coefficients throughout the loop. The theoretical predictions are validated against an extensive experimental dataset generated from a purpose-built loop thermosyphon test facility operated under controlled conditions simulating Lagos and Kano solar insolation profiles. Experimental validation demonstrates that the modified theory reduces the root mean square prediction error for overall thermal resistance from 31 percent in the benchmark correlations to 8 percent. Dimensionless design charts derived from the theory enable systematic optimisation of loop geometry for specific Nigerian climatic zones. Keywords: loop thermosyphon, two-phase heat transfer, buoyancy-driven flow, solar thermal, theoretical model.