Experimental study on the CHF characteristics with different coolant injection conditions and degassing effects on a downward-facing plane

• The critical heat flux (CHF) on a downward-facing plane is experimentally investigated.
• Shorter coolant injection distance to the plane can increase the CHF.
• Large coolant flow rate also increase the CHF.
• Degassing the coolant reduces the impurities and then increases the CHF.
• The bubble size is smaller with shorter coolant injection distance or the degas process.

The critical heat flux (CHF) of a downward facing heated surface has been a hot study topic heat flow in recent years. The various applications include nuclear, electronic cooling, fuel cells, steel-making and chemical reactors. Many studies have found that the CHF of the downward facing heated surface is lower than that of the upward facing heated surface because bubbles are easily accumulated at the heated surface by buoyancy. This study aims to investigate the CHF with different coolant injection distances, different coolant flow rates, and the degassing effect for the coolant. The result shows that shorter coolant inlet distance leads to better heat transfer effect, and vice versa. The flow rate also influences the CHF obviously, and the larger flow rate leads to better heat removal capability. The purpose of degassing is to reduce air concentration in the water, which causes premature CHF. The result also indicates that a degassed case has better CHF performance than the non-degassed case.