Incipient Boiling of Sodium in Seven-Pin Bundle under Forced Convection Conditions

Experimental studies were carried out on incipient boiling of sodium flowing in an electrically heated seven-pin bundle under forced convection.

In the first series of experiments temperature distributions in the bundle were measured under non-boiling conditions. The measured temperature distributions agreed fairly well with the calculation by the NORMAL code.

In the second series of experiments incipient-boiling (IB) phenomena were investigated, with particular reference to the IB superheat and the voiding patterns. The IB wall superheat decreased with increase in flow velocity. The observed coolant voiding was limited in the central subchannel because of the steep radial temperature gradient in the bundle. In order to describe this voiding process a two-dimensional voiding model was required.     

Local Sodium Boiling behind Local Flow Blockage in Simulated LMFBR Fuel Subassembly

This paper deals with local sodium boiling in the downstream of a six-subchannel blockage in an electrically heated LMFBR fuel subassembly mock-up.

The first series of experiments were conducted to measure temperature distributions in the downstream of the blockage under non-boiling conditions. The measured temperature rise due to the blockage agreed fairly well with the calculation by the LOCK code.

The second series of experiments were performed to investigate local boiling phenomena. In the local boiling region, no flow instability was observed since the sub-channels near the wrapper wall were still filled with sub-cooled liquid. In the nearly bulk boiling region, however, considerable upstream voiding occurred and then the inlet flow decreased, leading to final dryout.

The boiling caused a considerable increase in acoustic noise intensity. The root-mean-square (RMS) noise level of approximately 20 mbar obtained in the present local boiling experiments with sodium was much higher than that (approximately 0.5 mbar) in the ordinary nucleate boiling experiments with water. The peak observed in the hertz ranges was due to the repetition of bubble formation and collapse. In the kilohertz ranges, however, resonance peaks were superposed on a smooth curve with a broad peak at approximately 7 kHz.

The frequency (2.9 and 20.2 sec^?1) of bubble formation decreased with the increase of the bubble size at its point of maximum development. The product of the bubble frequency and the equivalent diameter was found to be constant.     

Experimental Studies on Heat Transfer by Natural Convection and Pool Boiling of Sodium in a Strong Magnetic Field

This report deals with an experiment on the heat transfer of liquid sodium, with particular reference to the effects brought by the application of a magnetic field on pool boiling. The test section, a heater pin of 6.5 mm diameter, was inserted vertically into the center of a sodium tank. The heating surface of the pin was parallel to the magnetic field as well as to the direction of gravity.

Under conditions of natural convection in a magnetic field, a sharp rise of the heating surface temperature was always seen to occur at some point when the heat flux was gradually increased, accompanied by the onset of sharp temperature oscillations.

The surface superheat required for the initiation of boiling decreased with increasing intensity of the applied magnetic field, reached a minimum, then increased again.

The surface temperature fluctuations in nucleate boiling was higher under magnetic field than when free of such influence. The critical heat flux for burn-out was not appreciably affected by magnetic field.