An experimental study was carried out to examine the time periodic evaporation heat transfer for refrigerant R-134a flowing in horizontal narrow annular ducts subjected to wall heat flux oscillation in the form of triangular waves. The experiment was performed for the gap of the annular duct delta = 1.0, 2.0 and 5.0 mm. Besides, the amplitude of the imposed heat flux oscillation Delta q was set at 0, 10, 30, 50, 80 and 100% of mean imposed heat flux (q) over bar. Moreover, the period of the heat flux oscillation t(p) was fixed at 2, 20, 30, 60,120 and 600 s. Attention was mainly paid to examining the effects of the amplitude and period of the wall heat flux oscillation on the time periodic evaporation heat transfer in the narrow annular pipe. The measured evaporation heat transfer data are expressed in terms of the evaporation heat transfer coefficient. The thermal characteristics of the oscillatory evaporation heat transfer were illustrated by showing the time variations of the instantaneous heated pipe wall temperature T-w, and evaporation heat transfer coefficient h(r). Measured results showed that the time-average heat transfer coefficients for the oscillatory evaporation heat transfer of R-134a were not affected to a noticeable degree by the amplitude and period of the imposed heat flux oscillation. Besides, the stronger wall temperature oscillation was noticed for a longer period and a larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux. (C) 2016 Elsevier Ltd. All rights reserved.