Effect of helium pressure on natural convection heat transfer in a prismatic dual-channel circulation loop

The effects of helium pressure on the convection heat transfer coefficient and temperature fields (helium and wall surface temperatures) in a unique scaled-down dual-channel natural circulation loop with upper and lower plena have been investigated in this study. Natural convection is one of the passive safety systems of the prismatic very high-temperature reactors (VHTRs) during the accident scenarios. The operating helium pressure was varied from 413.47 to 689.12 kPa in the temperature range from 6 to 196 °C. Radial and axial measurements were carried out along the flow channels using a new sophisticated flush wall-mounted heat transfer coefficient probe in conjunction with the radial adjuster for T-thermocouple that are integrated in a novel way to characterize natural convection heat transfer in terms of heat transfer coefficient, Nusselt number, helium temperature, and wall surface temperature. The obtained experimental results along the flow channels showed the dependence of natural convection on the system's pressure in which the Rayleigh number is proportional to the square of the helium pressure (Ra αP2). Also, it was found that upon increasing the helium pressure from 413.47 to 689.12 kPa, the heat transfer coefficient and Nusselt number are increased by 30% and 35%, respectively. Moreover, the wall surface temperature along the downcomer and riser channels are decreased by 12.7% and 18% with increasing the helium pressure from 413.47 to 689.12 kPa, respectively.