Cooling of Heated Solid Cylinder Supported on Bedded and Embedded Substrates by Impinging Air Jet

This study seeks to develop an experimental and modeling framework for characterizing cooling of a hot body that is supported on a substrate and cooled by a combination of jet impingement convection, radiation, and conduction into the substrate. While the radiation cooling is easily characterized, there are challenges in accurately specifying the convective and conductive cooling rates. The literature on jet impingement cooling is extensive, but there are significant differences in results using different correlations for nominally similar conditions. To characterize the cooling process for the multi-mode phenomenon, the convective heat transfer coefficient was measured for objects with diameter less than the jet diameter using the naphthalene sublimation analogy. Two different substrate configurations were tested. In one, the object completely rests on top of the substrate (bedded), while in the other, the object is partially subsumed within the substrate (embedded). After modeling the radiative and convective cooling components, a conductive resistance was inferred. The multi-mode model could then predict the cooling rate curve for a range of experimental conditions. Additionally, using the model, the relative contributions from the different modes of heat transfer could be determined. For the embedded configuration, initially most of the cooling was due to conduction. The majority of the losses are due to convection and radiation for the bedded configuration with calcium silicate. Two materials with higher thermal effusivities replaced the substrate in the bedded configuration. These cases had larger relative contributions to cooling from conduction compared to the bedded calcium silicate case.

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.

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.

Lateral Edge Effects on the Sherwood Number in Turbulent Flow Over a Flat Plate

Experimental results for the Sherwood number variation near the lateral edge of the active surface of a smooth, finite-width flat plate in turbulent boundary layer flow are presented. Using naphthalene sublimation, local mass transfer rates are found for two different free stream velocities. A semi-empirical correlation of the experimental data is presented, allowing calculation of the increase of mass transfer near the edge and the size of the region affected by the lateral edge. The effect is shown to scale more so with the diffusion thickness than the boundary layer thickness.

Local Heat and Mass Transfer Characteristics for Multi-Layered Impingement/Effusion Cooling

Multi-layered impingement/effusion cooling is an advanced cooling configuration that combines impingement jet cooling, pin cooling, and effusion cooling. The arrangement of the pins is a critical design factor because of the complex heat transfer in the internal structure. Therefore, it is important to measure the local heat transfer at all internal surfaces as a function of the pin spacing. In this study, a naphthalene sublimation method was employed to measure the details of the heat/mass transfer at the internal surfaces, including the injection plate, effusion plates, and the pins. An staggered array of holes was formed at the injection plate and effusion plates where the ratio of the height to the diameter of the pins, h/d, was fixed at 0.25. The ratio of the pin spacing to the diameter, sp/d, was varied in the range 1.5≤sp/d≤6, and the Reynolds number based on the hole diameter was 3000. As a result, a vortex ring formed near the pin, leading to re-impingement flows in the narrow channel. The jet flow impinged strongly on the pin, resulting in a large heat transfer region at each surface. The total average Sherwood number with sp/d=1.5 was larger than that with sp/d=6 by a factor of 1.5.