Experimental investigation on powder conductivity for the application to double wall heat exchanger (NACIE-UP)

Highlights: • Porous media provide temperature drop or enhance heat transfer in double wall tubes. • The conductivity of stainless steel powder has been assessed in the TxP facility. • The loading procedure and the thermal cycling affect the conductivity of the powder. • Grain growth occurs during thermal cycling and causes an increase of conductivity. • The data obtained fits the estimated conductivity based on NACIE facility's test. - Abstract: The double wall tube bundle heat exchanger is generally constituted by two or three (called double wall bayonet) concentric tubes. It is based on the concept to provide a double physical separation between two fluids: the coolant (i.e. water) and the hot fluid (i.e. lead). There are two primary reasons for the separation of the fluids. The first is to maintain a given temperature drop between the hot fluid and the coolant. The second is to increase the safety margin of the unit by reducing the probability of interaction between the coolant and the hot fluid. Furthermore, this configuration allows the possibility to monitor eventual leakages from the coolant or from the hot fluid by pressuring the separation region. On the other hand, if it is necessary to achieve high thermal performance of the unit, the annular space that separates the fluids should be filled with a heat transfer enhancer (i.e. sintetic diamond powder, stainless steel-SS powder). Several applications of the double wall and double wall bayonet tube heat exchangers have been designed and constructed at ENEA CR Brasimone. In particular, there are four facilities (NACIE and NACIE-UP, HELENA and HERO) whose heat exchanger is based on this concept and make use of SS powder (AISI-304 or AISI-316) to provide a temperature drop or to accommodate monitoring and heat transfer enhancement. In this framework, the Tubes for Powders (TxP) facility has been designed and constructed during 2012 and has been operating since the beginning of 2013 to investigate the conductivity of porous materials for their application in double wall heat exchangers. The facility consists of three concentric tubes and it allows to estimate the conductivity of a porous material by measuring the temperature drop across its borders and the removed power. It has the capability to introduce helium and to pressurize it up to 5 bar. The present paper focuses on the experimental campaigns carried out in TxP to characterize AISI-316 powder in support to the qualification of the NACIE-UP heat exchanger. The main goals of the experiments are to assess the conductivity of AISI-316 powder, to highlight the effects of the procedure adopted to load the powder in the facility on its conductivity and to investigate the influence of thermal cycling. The experimental findings are finally compared to the results obtained from experimental campaigns performed in NACIE and applied to NACIE-UP to develop and discuss the pre-tests calculations carried out by means of computational fluid dynamic code