Abstract
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.
Highlights
Convective heat transfer coefficients are often determined by complex experiments involving advanced instruments and difficult procedures
Naphthalene is favorable for mass transfer experiments because it possesses desirable properties such as low toxicity and a sublimation that occurs at room temperature
Even if the mass/heat transfer coefficient is obtained with an isothermal condition it can be applied to other boundary conditions, because the mass/heat transfer coefficient is a weak function of the wall temperature distribution in turbulent flows [1]
Summary
Convective heat transfer coefficients are often determined by complex experiments involving advanced instruments and difficult procedures. These procedures are even more difficult when the local heat transfer distribution over an entire test section in a practical engineering model with a complex geometry is needed. Naphthalene is favorable for mass transfer experiments because it possesses desirable properties such as low toxicity and a sublimation that occurs at room temperature It has good machining and casting properties. Mass transfer boundary conditions analogous to isothermal and adiabatic walls in convective heat transfer can be imposed in this method. The heat transfer coefficient, which is often desired, can be readily determined from the measured mass transfer results with good confidence via the mass/heat transfer analogy. The naphthalene sublimation technique has been successfully used for the study of the mass and heat transfer in a variety of applications, which include external flows, flow in ducts and channels, heat exchanger, heat transfer enhancement, electronic cooling and rotating transfer surfaces [1,2,3,4]
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