Pool boiling heat transfer.

Nucleate pool boiling heat transfer to slush and liquid hydrogen for SMES.

 When stored liquid or slush hydrogen is used as the refrigerant for SMES, it is important to consider nuclear pool boiling heat transfer properties.

 We used a 0.025 m diameter circular flat-plate made of copper as a heat transfer surface. The heat transfer surface was placed in differing orientations (horizontal upward facing, vertical, and horizontal downward facing) to investigate nuclear pool boiling heat transfer to liquid hydrogen at the normal boiling point (0.1 MPa, 20 K: NBP liquid), liquid hydrogen at the triple point pressure (0.007 MPa, 14 K: TP liquid), and settled slush hydrogen at the triple-point pressure (0.007 MPa, 14 K: TP slush). In the experiments, mass solid fractions of 20-35 wt.% were used, and the critical heat flux (CHF) was verified. Heat transfer was also similarly measured for liquid and slush nitrogen [25].

 The above figure presents heat transfer to NBP liquid, TP liquid, and TP slush in the case of the horizontal upward facing orientation. Heat flux q is represented on the vertical axis, while the temperature difference ∆T (superheat) is represented on the horizontal axis. Also, using the Kutateladze eq. applied to the horizontal upward facing orientation, calculated CHF values q* for liquid hydrogen, liquid nitrogen, and liquid helium are given.

 For NBP liquid, the heat transfer coefficient h (h = q /∆T) was about that of liquid helium, while the heat transfer coefficient for TP slush in the high heat flux region fell to approximately 0.5 times that of NBP liquid.

 When comparing CHF values q* = 11 W/cm2T = 2 K) for NBP liquid with q* = 5 W/cm2T = 2.5 K) for TP slush hydrogen and q* = 0.8 W/cm2T = 0.8 K) for liquid helium in the case of a horizontal upward facing orientation, NBP liquid hydrogen is characterized by the greatest heat transfer coefficient and the highest CHF.

 For transport via pipeline to a remote location, because heat inleak during such transport would melt the solid hydrogen particles, it is predicted that storage would be in the form of liquid hydrogen. In this case, the use of liquid hydrogen as the refrigerant in a SMES system would be advantageous from the standpoint of heat transfer properties.

 It is also noteworthy that, as shown in the above figure for liquid nitrogen, CHF q* = 20 W/cm2T = 6 K) is about twice as large as for liquid hydrogen, while at the same heat flux, the heat transfer coefficient falls to a level of 0.4 times.

 The experimental results for the TP slush hydrogen and nitrogen were compared with the Rohsenow’s eq. (refer to the figure below) [31], and consideration was given as to applicability with respect to a solid-liquid two-phase fluid. The coefficients Csf and s are values determined by the combination of the heat transfer surface material and the liquid, with the same value being applicable regardless of differences in the pressure of liquid. The thermo-physical values of the fluid were used with respect to the NBP and the TP states.
 First, after determining Csf and s so as to satisfactorily express the actual experimental results for the NBP liquid (Csf = 0.010, s = 1.0 for hydrogen and Csf = 0.0065, s = 1.7 for nitrogen), the heat transfer coefficients for the TP liquid and the TP slush were predicted. Given the difficulty involved in the experimental work, it is important from a practical standpoint to be able to predict the heat transfer coefficient for solid-liquid two-phase slush using the experimental results for the NBP liquid. In the case of slush, the value used for the heat of vaporization λ includes the heat of fusion for the solid. The calculation results for the TP liquid and the TP slush are represented by the solid line in the figure below.
 As a result of predicting the heat transfer coefficient for the TP slush hydrogen and nitrogen using experimental results of the NBP liquid hydrogen and nitrogen, and the Rohsenow’s eq., somewhat lower values of heat transfer coefficient for the TP slush hydrogen were obtained than those of the experimental results. However, these values were found to have a satisfactory fit to the experimental data.

 Comparing the CHF values for the NBP liquid hydrogen and the TP slush hydrogen with that of the NBP liquid helium, it can be seen that the former values are quite high, meaning thatliquid or slush hydrogen can be reasonably expected to be used as the refrigerant for superconducting machines using high-temperature superconducting material (MgB2).