Onset and departure of flow boiling heat transfer characteristics of cyclohexane in a horizontal minichannel

• Nucleation hysteresis for cyclohexane was observed in minichannels.
• Wall temperature slightly decreased before boiling heat transfer deterioration.
• The wall temperature decrease occurred at a wide vapor quality range of 0 < x < 1.
• The wall temperature decrease did not exceed 10 °C.
• The wall temperature decrease phenomenon was suppressed by the increasing pressure.

The flow boiling behavior of cyclohexane was experimentally investigated in a horizontal minichannel with inner diameter of 2.0 mm. The nucleation hysteresis phenomena at onset of nucleate boing (ONB) and characteristics at departure of flow boiling were focused on. Both the temperature overshoot due to nucleation hysteresis and wall superheat to maintain boiling conditions decreased with the increasing pressure. The nucleation hysteresis can be clearly observed at pressure of (1.0 and 2.0) MPa, but disappeared at 3.0 MPa. A wall temperature decrease phenomenon (TDP) was found to occur just before flow boiling heat transfer deterioration (HTD). The magnitudes of TDP would be less than 10 °C, but bigger than the temperature overshoot of about 3 °C at ONB. Wherever HTD happened at the saturation boiling conditions with high or low vapor quality ranging from 0 to 1, the TDP were observed, but suppressed by the increasing pressure. Accordant with nucleate hysteresis, the TDP were clearly recorded at P = 1.0 and 2.0 MPa, but disappeared at 3.0 MPa. The mechanism of TDP before HTD at flow boiling condition, a problem needed extensive focus investigation, may give a better understanding or even a universe CHF mechanism for both dryout and burnout conditions.