Shi-Chune Yao

Mechanisms of film boiling heat transfer of normally impacting spray

Abstract The heat transfer mechanisms of horizontally impacting sprays were studied experimentally. An impulse-jet liquid spray system and a solid particle spray system were used. The liquid spray system is capable of producing uniform droplets with the independent variables of droplet size, velocity, liquid flow rate, and air velocity. The horizontally impacting sprays give a lower heat transfer at film boiling than the corresponding vertically impacting spray. The film boiling heat transfer is mainly controlled by the liquid mass flux. At low liquid mass flux and low droplet Weber number, the heat transfer increases with the droplet Weber number. At high droplet Weber number or high liquid mass flux, the heat transfer is not significantly affected by the droplet Weber number.

Pool boiling heat transfer in a confined space

Abstract Pool boiling heat transfer in a confined space has been studied for vertical narrow annuli with closed bottoms. Experiments were performed for Freon-113, acetone, and water at 1 atm. for annuli with heights of 25.4 and 76.2 mm, and gap sizes of 0.32, 0.80 and 2.58 mm. Three boiling regimes are identified through visual observation. The importance of the Bond number is also demonstrated. When the Bond number is less than unity, the isolated deformed bubble regime occurs at low heat flux and the coalesced deformed bubble regime is observed at high heat flux. For Bond numbers slightly larger than unity at high heat flux, nucleation on the heated surface has been observed. The general behavior of the boiling curves and the heat transfer mechanisms of these boiling regimes are presented and discussed.

Heat-Transfer Augmentation in Rod Bundles Near Grid Spacers

Heat-transfer augmentation by straight grid spacers in rod bundles is studied for single-phase flow and for post-critical heat flux dispersed flow. The heat transfer effect of swirling grid spacers in single-phase flow is also examined. Governing heat-transfer mechanisms are analyzed, and predictive formulations are established. For single-phase flow, the local heat transfer at a straight spacer and at its upstream or downstream locations are treated separately. The effect of local velocity increasing near swirling spacer is considered. For post critical heat flux (CHF) dispersed flow, the heat transfer by thermal radiation, fin cooling, and vapor convection near the spacer are calculated. The predictions are compared with experimental data with satisfactory agreement.

A chip scale electrocaloric effect based cooling device

The recent finding of large electrocaloric effect in several ferroelectric polymers creates unique opportunity for developing compact size solid state cooling cycles beyond the traditional mechanical vapor compression cycles. Here, we show that, by employing regeneration process with solid state regenerators, a chip scale Electrocaloric Oscillatory Refrigeration (ECOR) can be realized. A prototype ECOR is fabricated and characterized. More than 6 K temperature span is obtained near room temperature between the hot and cold sides of a 2 cm long device. Finite volume simulation validates the test results and shows the potential high performance of the ECOR.

Analysis on film boiling heat transfer of impacting sprays

Abstract The analysis on film boiling of impacting sprays is separated into two cases—a dilute spray (negligible heat transfer interaction among droplets) and a dense spray (significant interaction). The heat transfer of an impacting dilute spray is analyzed by dividing the mechanisms into three identified subprocesses—drop contact heat transfer, bulk air convective heat transfer and radiative heat transfer. Both horizontal and vertical sprays are modeled. The predictions are very satisfactory. For the dense spray film boiling, the asymptotic approach is successful. The low end asymptote corresponds to the dilute spray situation while the high end asymptote is adequately represented by the pool boiling situation for most spray conditions. The liquid mass flux is the most dominant parameter in both cases. Dense spray film boiling is found to show very little dependence on droplet parameters; however, dilute spray film boiling is significantly influenced by droplet parameters.

The dynamics and leidenfrost temperature of drops impacting on a hot surface at small angles

Abstract The dynamics of water drops impacting at small angles on hot surfaces were studied. Experiments were conducted using a monosize droplet stream and a rotating disk. At normal impact, a drop will disintegrate when its Weber number is high. It is observed in this study that at small-angle impact the existence of tangential relative velocity tends to destabilize the drop. On the other hand, this tangential relative velocity reduces the chance of physical contact between the drop and the surface during impact. Therefore, when the impact angle is decreased the Leidenfrost temperature is reduced. Correlations are established for the description of this behavior.

Correlation of Thermal Conductivities of Unidirectional Fibrous Composites Using Local Fractal Techniques

The arrangement of fibers strongly influences heat conduction in a composite. Traditional approaches using unit cells to describe the fiber arrangements work well in the case of ordered arrays, but are not useful in the context of disordered arrays, which have been analyzed in the literature by statistical means. This work presents a unified treatment using the tool of local fractal dimensions (although, strictly speaking, a composite cross section may not be an exact fractal) to reduce the geometric complexity of the relative fiber arrangement in the composite. The local fractal dimensions of a fibrous composite cross section are the fractal dimensions that it exhibits over a certain small range of length scales. A generalized unit cell is constructed based on the fiber volume fraction and local fractal dimensions along directions parallel and transverse to the heat flow direction. The thermal model resulting from a simplified analysis of this unit cell is shown to be very effective in predicting the conductivities of composites with both ordered as well as disordered arrangement of fibers. For the case of square packing arrays, the theoretical result of the present analysis is identical to that of Springer and Tsai.

SPRAY COOLING OF HIGH TEMPERATURE METALS USING HIGH MASS FLUX INDUSTRIAL NOZZLES

Systematic experiments were conducted for the spray cooling of high-temperature stainless steel using three different types of industrial sprays: full cone and flat hydraulic nozzles and a flat air-mist nozzle. In the present study, a wide range of mass flux (1.5–30 kg/m2sec) is covered, which has never been thoroughly investigated before. Orientations with respect to gravity and spray angle were also explored. The data have been analyzed and correlated. The results of this study will be important to the product quality control in industries by providing a good estimation of heat flux at different mass flux, spray types, and surface temperatures, especially for spray cooling of stainless steel and other metals with similar thermal properties.

Microelectromechanical System-Based Evaporative Thermal Management of High Heat Flux Electronics

We describe the development of embedded droplet impingement for integrated cooling of electronics (EDIFICE), which seeks to develop an integrated droplet impingement cooling device for removing chip heat fluxes over 100 W/cm 2 , employing latent heat of vaporization of dielectric fluids. Micromanufacturing and microelectromechanical systems are used as enabling technologies for developing innovative cooling schemes. Microspray nozzles are fabricated to produce 50-100 pm droplets coupled with surface texturing on the backside of the chip to promote droplet spreading and effective evaporation. We examine jet impingement cooling of EDIFICE with a dielectric coolant and the influence of fluid properties, microspray characteristics, and surface evaporation. The development of micronozzles and micro structured surface texturing is discussed. Results of a prototype testing of swiss-roll swirl nozzles with dielectric fluid HFE-7200 on a notebook PC are presented

Forced convective boiling in horizontal tube bundles

Abstract Experimental studies of crossflow boiling on a horizontal tube at various in Mass fluxes, local flow qualities and geometric arrangements are investigated. Since abundant information is available for the boiling on a single tube in a pool but it is still not clear whether this information in ay be applicable to tubes in bundles, the present study is therefore performed on three different conditions, namely: (1) a heated tube in a channel; (2) a heated tube in a non-heated, in-line tube bundle; and (3) a heated tube in a heated, in-line tube bundle. The different heat transfer results between a single tube in a channel and a tube in a non-heated bundle, and between a non-heated bundle and a heated bundle are discussed in terms of the different flow field geometry and thermal environment respectively due to the presence of different structures and the heating conditions near the tube. A modified Chens correlation is established to predict the heat transfer of a single tube in a channel or in a bundle. The correlation is also in good agreement with other data in the literature.