Experimental investigations of boiling heat transfer of subcooled ammonia in OTEC plate heat exchangers

Laboratory experiments were conducted to investigate the boiling heat transfer characteristics inside a plate heat exchanger. Water and ammonia, serving as working fluids for Ocean Thermal Energy Conversion (OTEC) systems, were employed. This study systematically evaluates the effects of four key thermal-hydraulic parameters: water inlet temperature (21.70 °C–30.11 °C), ammonia inlet temperature (6.71 °C–12.30 °C), ammonia mass flux (11.69 kg/m 2 ·s–23.31 kg/m 2 ·s), and water flow rate (2.39 L/min–6.31 L/min). Notably, these operating conditions lie within relatively low parameter ranges compared to prior studies. Entering the heat exchanger in a subcooled state, the ammonia undergoes both single-phase subcooled heating and two-phase boiling in the channel. Under the present conditions, the ammonia boiling process remains exclusively within the bubbly flow regime. A new data reduction methodology was developed to extract the heat transfer coefficients and key flow variables from the raw experimental data. The calculated results demonstrate that the two-phase boiling regime achieves significantly higher heat transfer performance than the subcooled single-phase regime. In parallel, flow visualisation experiments were performed on the ammonia channel. Flow visualisation analysis yielded the instantaneous vapour velocity, while the local liquid velocity was inferred using a linear-growth heat transfer model. The results reveal that the ammonia vapour bubbles travel substantially faster than the liquid phase near the channel exit, and the velocity ratio reaches approximately 7 for one representative case.