Study On Convection Heat Transfer Characteristics Of Pebble-bed With Internal Heat Source

This paper deal with the experiment study on single-phase forced convection heattransfer and subcooled boiling in pebble-bed channel with internal heat source. Theexperimental study was carried out in a quartz glass tube with the diameter of75mm and aheight of980mm. This tube served as the model of pebble-bed channel with internal heatsource. Distilled water was used as coolant. In the channel, a number of carbon steel ballswith the diameter of8mm were placed. We used electromagnetic heater as simulative internalheat source to heat the steel balls.On the basis of the model, the experiment studied several factors that influenced the heattransfer characteristics, using abundant experimental data such as flow rate and temperature,etc. The experimental result showed that the Re number was a major that influenced the heattransfer coefficients. The heat transfer coefficients increased substantially with the increase ofRe number. The inlet temperature of the coolant was also an important factor of the heattransfer coefficients. The heat transfer coefficients were higher with lower inlet coolanttemperature. When there was low flowrate, there was an obvious entrance effect. But it wasweakened quickly with the increase of the Re number. The increase of heated power slightlyreduced the coefficients of the heat transfer, but the change was subtle. It was only reduced1%in the stage of low Re number. On the basis of experimental data, the dimensionlesscorrelation formula of heat transfer law on pebble-bed channel with internal heat source wasfound. The deviations between calculation and experimental values were within±17%.This paper also studies the feature of ONB (onset of nucleate boiling) in pebble-bedchannel with internal heat source. This paper used the superheat of wall’s point of inflexionon curve to fix the ONB. The factors that influenced ONB were analyzed at the velocityranging from0.3m3/h to1.0m3/h, heat flux ranging from5to77KW/m（2voltage from180V to500V）, and inlet temperature with70℃、80℃、85℃respectively. Analysis showed thatthe superheat of wall increased linearly with the increase of heat flux. But the increase wasslow and even stopped when the ONB took place. The ONB need higher superheat of walland higher heat flux to take place with the increasing of the Re number. But the increasing Renumber did not influence the curve of superheat with heat flux when the ONB occurred. Thesuperheat of wall and heat flux required by ONB was smaller when the location was closer to the exit. The higher inlet temperature needs the lower heat flux when the ONB occurred, but itdid not influence the superheat of wall. When the coolant entered into the phase of thenucleate boiling, the deferent inlet temperature did not influence the curve of superheat withheat flux.This paper also studies the boiling hysteretic phenomenon. The boiling curves wereobtained at the increasing and decreasing of the heat flux, which made it possible to detect theboiling hysteretic phenomena. The boiling hysteretic phenomenon has been observed in thepebble bed was inconspicuous by many times of experiments. According to the analysis of theexperimental results, we have a further understanding phenomenon of boiling hysteretic in thepebble-bed channel.