Assessment of the naphthalene sublimation technique for determination of convective heat transfer in fundamental and industrial applications

Artikel i vetenskaplig tidskrift, 2020

The naphthalene sublimation technique is an experimental method for indirectly determining convective heat transfer. The technique is here assessed for two different configurations: the local heat transfer distribution for a circular air jet impinging normal to a flat surface, and the heat transfer occurring in the stator core of an electric generator model. The turbulent impinging jet is fully developed. Two Reynolds numbers based on the nozzle exit condition, 15000 and 23000, and two nozzle diameter distances from the jet exit to the surface, 6 and 8, are considered. For the generator turbulent internal flow with Reynolds number of 4100 is considered, based on the hydraulic diameter of stator ventilation ducts. Modern surface scanning methods and imprints
of the naphthalene specimens were used for measuring the naphthalene sublimation rate. The impinging jet results are compared with experimental data found in the literature. Results from the generator model and numerical simulations are compared. For the impinging jet, the results show agreement with the already published experimental data sets. For the generator model, heat transfer results from experiments differ by around 13% compared to numerical results if a scanning of the surface is used for measuring the naphthalene sublimation and around 5% if weights are used for measuring the sublimation rate. Therefore, the results depend on the way the sublimation rate is quantified. From this study, it is possible to affirm that with advanced scanning procedures, the heat transfer can be resolved with very small naphthalene sublimation in cases of both fundamental and complex
industrial applications such as electric generators.