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

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.

Characterization of Impingement Heat/Mass Transfer to the Synthetic Jet Generated by a Biomimetic Actuator

Two biomimetic synthetic jet (SJ) actuators were designed, manufactured, and tested under conditions of a jet impingement onto a wall. Nozzles of the actuators were formed by a flexible diaphragm rim, the working fluid was air, and the operating frequencies were chosen near the resonance at 65 Hz and 69 Hz. Four experimental methods were used: phase-locked visualization of the oscillating nozzle lips, jet momentum flux measurement using a precision scale, hot-wire anemometry, and mass transfer measurement using the naphthalene sublimation technique. The results demonstrated possibilities of the proposed actuators to cause a desired heat/mass transfer distribution on the exposed wall. It was concluded that the heat/mass transfer rate was commensurable with a conventional continuous impinging jets (IJs) at the same Reynolds numbers.

Sublimation Rate of Vertically Oriented Naphthalene Cylinders in Natural Convection

The naphthalene sublimation technique was used to determine the rate of mass transfer from three solid naphthalene cylinders in a natural convection environment. The cylinder diameters measured 2.5 cm (1 in), 3.8 cm (1.5 in), and 5 cm (2 in) nominally. Sublimation rates were measured and the mass transfer coefficients were calculated. Correlations were developed for the Sherwood vs. Rayleigh numbers, Sherwood vs. Grashof numbers, and mass transfer coefficient vs. diameter.

Modeling and Measurements of Heat/Mass Transfer in a Linear Turbine Cascade

Measurements of the mass/heat transfer coefficients on the blade and end wall surfaces of a linear turbine cascade are compared to numerical predictions using the standard shear stress transport (SST) closure and the SST model in combination with the Reθ–γ transition model (SST-TRANS). Experiments were carried out in a wind tunnel test section composed of five large-scale turbine blades, using the naphthalene sublimation technique. Two cases were tested, with exit Reynolds number of 600,000 and inlet turbulence values of 0.2% and 4%, respectively. The main secondary flow features, consisting of the horseshoe vortex system, the passage vortex, and the corner vortices, are identified and their influence on heat/mass transfer is analyzed. Numerical simulations were carried out to match the conditions of the experiments. Results show that large improvements are obtained with the introduction of the Reθ–γ transition model. In particular, excellent agreement with the experiments is found, for the whole spanwise extension of the blade, on the pressure surface. On the suction surface, performance is very good in the highly three-dimensional region close to the end wall, but some weaknesses appear in predicting the location of transition in the two-dimensional region. On the end wall surface, the SST model in combination with the transition model produces satisfactory results, greatly improved compared to the standard SST model.

Heat-Transfer Characteristics of a Non-Rotating Two-Pass Rectangular Duct With Various Guide Vanes in the Tip Turn Region

The present study investigated convective heat transfer inside a two-pass rectangular duct with guide vanes in the turning region. The objective was to determine the effect of the guide vanes on blade-tip cooling. The duct had a hydraulic diameter (Dh) of 26.67 mm and an aspect ratio (AR) of 5. The duct inlet width was 80 mm, and the distance between the tip of the divider and the tip wall of the duct was also 80 mm. Various guide vane configurations were used in the turning region. The Reynolds number (Re), based on the hydraulic diameter, was held constant at 10,000. The naphthalene sublimation technique was used to determine the detailed local heat-transfer coefficients, using the heat-and mass-transfer analogy. The results indicate that guide vanes in the turning region enhance heat transfer in the blade-tip region. The guide vane on the second-pass side of the turning region had higher heat transfer than the guide vane on the first-pass side. Strong secondary flow enhanced heat transfer in the blade-tip region. Dean vortices induced by the guide vanes pushed the high-momentum core flow toward the tip wall, and heat transfer was increased in the turning region, but decreased in the second passage. Consequently, a guide vane on the second-pass side of the turning region generates high-heat-transfer rates on the tip surface, and can also increase the thermal performance factor in a two-pass duct.

Heat Transfer Characteristics of a Non-Rotating Two-Pass Rectangular Duct With Various Guide Vanes in the Tip Turn Region

The present study investigated convective heat transfer inside a two-pass rectangular duct with guide vanes in the turning region. The objective was to determine the effect of the guide vanes on blade tip cooling. The duct had a hydraulic diameter (Dh) of 26.67 mm and an aspect ratio (AR) of 5. The duct inlet width was 80 mm, and the distance between the tip of the divider and the tip wall of the duct was also 80 mm. Various guide vane configurations were used in the turning region. The Reynolds number (Re), based on the hydraulic diameter, was held constant at 10,000. The naphthalene sublimation technique was used to determine the detailed local heat transfer coefficients, using the heat and mass transfer analogy. The results indicated that guide vanes in the turning region enhanced heat transfer in the blade tip region. The guide vane on the second-pass side of the turning region had higher heat transfer than the guide vane on the first-pass side. Strong secondary flow enhanced heat transfer in the blade tip region. Dean vortices induced by the guide vanes pushed the high-momentum core flow towards the tip wall, and heat transfer was increased in the turning region, but decreased in the second passage. Consequently, a guide vane on the second-pass side of the turning region generates high heat transfer rates on the tip surface, and can also increase the thermal performance factor in a two-pass duct.

Heat Transfer in Rotating Channel With Inclined Pin-Fins

This study is to examine experimentally the effects of pin inclination and pin height-to-diameter ratio on the heat/mass transfer characteristics in a pin-fin channel with and without rotation. The test model consists of staggered pin-fin arrays with an interpin spacing of 2.5 times of the pin-diameter (S/D=2.5) in both longitudinal and transverse directions. Detailed local heat/mass transfer coefficients on the two principal surfaces of rotating channel are measured using the naphthalene sublimation technique. The inclined angles (θ) studied are 60 deg and 90 deg. The pin height-to-diameter ratio (Hp/Dp) ranges from 2 to 4. The Reynolds number is fixed at 7.0×103 with two rotation numbers (0.0 and 0.2). The measured data show that the overall array heat/mass transfer decreases with the angle of inclination relative to the vertical orientation. The overall array averaged as well as the row-resolved heat/mass transfer increases with an increase in Hp/Dp. Rotation generally results in higher heat/mass transfer than the corresponding stationary case. The nonuniformity or redistribution of heat/mass transfer induced by the Coriolis force generally perceived in a ribbed or smooth channel is less evident in a pin-fin channel.

Effect of Wake-Disturbed Flow on Heat (Mass) Transfer to a Turbine Blade

This study investigates the effect of wakes in the presence of varying levels of background freestream turbulence on the heat (mass) transfer from gas turbine blades. Measurements using the naphthalene sublimation technique provide local values of the mass transfer coefficient on the pressure and suction surfaces of a simulated turbine blade in a linear cascade. Experimental parameters studied include the pitch of the wake-generating blades (vanes), blade-row separation, Reynolds number, and the freestream turbulence level. The disturbed flow strongly affects the mass transfer Stanton number on both sides of the blade, particularly along the suction surface. An earlier transition to a turbulent boundary layer occurs with increased background turbulence, higher Reynolds number, and from wakes shed from vanes placed upstream of the linear cascade. Note that once the effects on mass transfer are known, similar variation on heat transfer can be inferred from the heat/mass transfer analogy.