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Experimental studies on mist/steam cooling in a heated horizontal tube have been performed. Wall temperature distributions have been measured under various main steam flow rates, droplet mass ratios, and wall heat fluxes. Generally, the heat transfer performance of steam can be significantly improved by adding mist into the main flow. An average enhancement of 100% with the highest local heat transfer enhancement of 200% is achieved with 5% mist. When the test section is mildly heated, an interesting wall temperature distribution is observed: the wall temperature increases first, then decreases, and finally increases again. A three-stage heat transfer model with transition boiling, unstable liquid fragment evaporation, and dry-wall mist cooling, has been proposed and has shown some success in predicting the wall temperature of the mist/steam flow. The PDPA measurements have facilitated better understanding and interpreting of the droplet dynamics and heat transfer mechanisms. Furthermore, this study has shed light on how to generate appropriate droplet sizes to achieve effective droplet transportation, and has shown that it is promising to extend present results to a higher temperature and higher pressure environment.

The present study formulates an improved approach for analyzing separated-flow transition that differentiates between the transition process in boundary layers that are laminar at separation and those that are already transitional at separation. The paper introduces new parameters that are necessary in classifying separated-flow transition modes and in accounting for the concomitant evolution of transition in separated shear layer and the average effect of periodic separation bubble build-up and vortex shedding. At least three separated-flow transition modes are positively distinguished: (a) transitional separation, with the transition starting upstream of the separation point and developing mostly as natural transition, (b) laminar separation/short bubble mode, with the onset of transition induced downstream of the separation point by inflexional instability and with a quick transition completion, and (c) laminar separation/long bubble mode, with the onset of transition also induced downstream of the separation point by inflexional instability, and with the transition completion delayed. Passing from one mode to another takes place continuously through a succession of intermediate stages. The location of maximum bubble elevation has been proved to be the controlling parameter for the separated flow behavior. It was found that, downstream of the separation point, the experimental data expressed in terms of distance Reynolds number Rex can be correlated better than momentum or displacement thickness Reynolds number. For each mode of separated-flow transition, the onset of transition, the transition length, and separated flow general characteristic are determined. This prediction model is developed mainly on low free-stream turbulence flat plate data and limited airfoil data. Extension to airfoils and high turbulence environment requires additional study.

Internal mist/steam blade cooling technology has been considered for the future generation of Advanced Turbine Systems (ATS). Fine water droplets of about 5 μm were carried by steam through a single slot jet onto a heated target surface in a confined channel. Experiments covered Reynolds numbers from 7500 to 25,000 and heat fluxes from 3 to 21 kW/m2. The experimental results indicate that the cooling is enhanced significantly near the stagnation point by the mist, decreasing to a negligible level at a distance of six jet widths from the stagnation region. Up to 200 percent heat transfer enhancement at the stagnation point was achieved by injecting only ∼1.5 percent of mist. The investigation has focused on the effects of wall temperature, mist concentration, and Reynolds number.

A numerical model is developed to determine convective heal transfer distributions for an array oj submerged planar jets impinging on a uniform heal flux or constant temperature moving surface where the surface motion is directed perpendicular to the jet planes. Unlike prior related work, the model accounts for the interaction between neighboring jets in the array without imposition oj an imposed cross flow. The effects oj surface speed, nozzle separation distance within an array, nozzle height, and Reynold.> number on the flow field, coefficient ojfriction, and Nusselt number are investigated. Results suggest that negkcting surface motion effects can lead to overestimates of heat transfer.