Flow boiling heat transfer of water in a thermosyphon loop based natural circulation solar steam generation system with a horizontally-arranged receiver towards parabolic trough collector (PTC) applications was studied experimentally. The diameter ratio (Rd) of the receiver was varied over a range from 1.0 to 2.0 to explore its effect on flow boiling heat transfer, especially the stratified flow boiling characteristics in the receiver tube, and thermal performance of the thermosyphon system. The results show that flow boiling heat transfer in the receiver was improved since Rd was increased to 1.2 due to a flow pattern transition which eliminated the effect of gas liquid stratification on surface wettability deterioration in the receiver. An optimal tube diameter ratio was found to be 1.4 with a heat transfer coefficient enhancement by 12.2% in comparison to the baseline case. Simultaneously, the rise of Rd resulted in an enhancement of backflow in the inlet of receiver. The effect of backflow on isothermal improvement in the falling tube of thermosyphon, however, was found to be irrelevant to the total heat pipe thermal resistance (R-HP). (C) 2015 Elsevier Ltd. All rights reserved.