Nengli Zhang

Innovative heat pipe systems using a new working fluid

The operating temperature and the heat load of capillary-pumping heat-pipe systems are much lower than maximum attainable limit, and the systems suffer operational instability problems because the working fluid has a negative surface-tension gradient with temperature that is unfavorable for the spreading or re-wetting on a heated surface. The present paper suggests an innovation of heat-pipe system by using new working fluids, which have a positive gradient of surface tension with temperature, to improve the performance of capillary-pumping heat-pipe systems and their operative stability.

Mechanisms of convection instability in thin liquid layers induced by evaporation

Abstract Available theories, including those of Rayleigh and Pearson, successfully explain convection in a thin liquid layer heated from below. However, the theories are unable to explain convection in an evaporating liquid layer heated/cooled from below. In this paper new mechanisms for convection instabilities in evaporating liquid layers are described from a physical viewpoint. Circulating flows in evaporating layers are shown to differ from the flow in layers heated from below without evaporation. Modified Marangoni and Rayleigh numbers are proposed to gauge the convection stability status in both evaporating layers and in liquid layers heated from below without evaporation.

Models for enhanced boiling heat transfer by unusual Marangoni effects under microgravity conditions

Abstract A new approach is suggested to enhance boiling heat transfer through introduction of unusual surface tension effects. The surface tension of aqueous solutions of alcohols with a chain length longer than four carbon atoms offers a positive gradient with temperature when the temperature exceeds a certain value. Moreover, the positive gradient near the boiling point has a very large value. This will generate a considerable driving force for bubble departure. As a result, in the nucleate boiling of these solutions, the Marangoni effect around the bubble surface will not impede the bubble detachment from the heater surface but rather will provide an additional driving force for the bubble departure. This effect combines with the buoyancy under normal gravity and acts as a main driving force of bubble departure in microgravity. Models for predictions of the bubble detachment diameters, the nucleate boiling heat transfer coefficient, and the critical heat flux are developed.

A new laser shadowgraphy method for measurements of dynamic contact angle and simultaneous flow visualization in a sessile drop

A new laser shadowgraphy method is presented to measure the dynamic contact angle of a sessile drop on a nontransparent metal substrate and simultaneously visualize flow motions inside the drop. A collimated laser beam is refracted into the drop, then reflected on the substrate surface and finally refracted out of the drop to form a shadowgraphic image on a screen. The instant diameters of the refracted-shadowgraphic image, cooperated with the corresponding instant contact-diameters of the drop measured from the magnified top view, are used to determine the instant contact angles of the sessile drop. At the same time, flow motions, if any, in the drop can be visualized from the refracted-shadowgraphic image. The new method is demonstrated to be a very simple, accurate, and unique optical technique for simultaneous measuring of the dynamic contact angle of a liquid drop spreading on a nontransparent metal substrate with flow visualization in the drop.

Nucleate Pool Boiling on Copper-Graphite Composite Surfaces and Its Enhancement Mechanism

An experimental study is performed on nucleate pool boiling in a highly wetting liquid, Freon-113, and a moderately wetting liquid, water, on copper‐graphite (Cu‐Gr) composite surfaces under atmospheric conditions. Boiling heat-flux data on the Cu‐Gr surfaces are compared with those on a pure copper surface. Best-fit correlation equations are derived to relate the boiling heat flux to wall superheat and boiling heat-transfer coefficient with the heat flux for each liquid‐solid combination. It is disclosed that the nucleate boiling heat transfer in both test liquids is augmented by the presence of graphite fibers and that the composite surface with 25% volume fraction of graphite fibers in the copper matrix exhibited the best enhancing performance in both test liquids. Water is characterized by a stronger enhancement and less degeneracy of the enhancement with an increase in the heat flux than Freon-113. The enhancement mechanism is proposed using a bubble departure model developed by the authors.

Effects of evaporation and thermocapillary convection on volatile liquid droplets

Results of an experimental investigation of evaporating sessile drops on a glass-slide surface for three volatile liquids show that both evaporation and thermocapillary convection in the sessile drop strongly affect the drop spreading and contact angle. The evolution of contact diameter of the drops can be divided into four stages: 1) initial spreading, 2) spreading-evaporation balance, 3) evaporation-dominating contraction, and 4) final rapid contraction. Molecular-kinetic spreading always occurs in the early first stage and is rapidly restrained and then taken over by the effects of evaporation. Thermocapillary convection, induced by the evaporation, promotes the competition of evaporation over the spreading and shortens the spreading-evaporation balance stage to become undetectable. Evaporation may increase or decrease the contact angle of the evaporating sessile drops, depending on the evaporation rate

Enhancements of Nucleate Boiling and Critical Heat Flux Under Microgravity Conditions

Two means are presented for enhancing nucleate boiling and critical heat flux under microgravity conditions: using microconfigured metal-graphite composites as the boiling surface and using dilute aqueous solutions of longchain alcohols as the working fluid. In the former, thermocapillary force induced by the temperature difference between the graphite-fiber tips and the metal matrix plays an important role in bubble detachment. Thus, boiling heat transfer performance does not deteriorate in a reduced-gravity environment. In the latter case, the surface-tension-temperature gradient of the long-chain alcohol solutions turns positive as the temperature exceeds a certain value. Consequently, the Marangoni effect does not impede, but rather aids in bubble departure from the heating surface. This feature is most favorable in microgravity conditions. As a result, the bubble size of departure is substantially reduced at higher frequencies. Based on the existing experimental data, and a two-tier theoretical model, correlation formulas are derived for nucleate boiling on the copper-graphite and aluminum-graphite composite surfaces, in both the isolated and coalesced bubble regimes. In addition, performance equations for nucleate boiling and critical heat flux in dilute aqueous solutions of long-chain alcohols are obtained.

Flow and Heat Transfer in Hydraulic Reservoir of Thrust Vector Control System

Launch vehicle thrust vector control provides vehicle steering functions during powered flight starting from liftoff until engine cutoff. The thrust vector control on the first stage of Ares 1 provides steering during the first 133 s of ascent, while the thrust vector control on the upper stage provides steering for the next 465 s. Heat absorption by the hydraulic reservoir is a key factor in the performance of the upper stage thrust vector control system. A computational fluid dynamics simulation of the fluid flow and heat transfer in the hydraulic reservoir in the Ares 1 upper stage thrust vector control has been performed with the corresponding operating conditions and environmental conditions. The reservoir was set at 75 % full of working fluid. Two steady-state cases, for fluid inlet temperatures of 294.3 and 394.3 K, and one transient case for the fluid inlet temperature ramping from 294.3 to 352.2 K over a 4.63 min period were simulated. The temperature, velocity, and pressure fields, as well as the absorbed heat for each case, were obtained, which have improved the understanding of the thermal dynamics of the thrust vector control subsystem.

TWO BASIC MODES OF BUBBLE GROWTH AND DETERMINATION OF DEPARTURE DIAMETERS IN POOL BOILING

Two basic modes are discovered to describe the bubble formation, growth, and departure in different liquids in pool boiling. Different physical models of bubble growth and departure in highly wetting liquids, such as Freon-12, and in moderately wetting liquids like water are suggested, and subsequently, the corresponding mathematic descriptions are presented. The prediction results for the departure diameters both in highly wetting liquids and in water agree well with the existing experimental data. Applying the proposed analysis method, the enhancement mechanism of pool boiling heat transfer on the metal-graphite composite surfaces is successfully explained.

Reduction of boiling thermal hysteresis in immersed electronics cooling on micro-configured graphite-metal conposite surfaces

It is known that hysteresis is an important and unique dynamic characteristics in two-phase systems in motion with or without rotation. In pool boiling which generates vapor phase, the system is set in motion due to a pumping action near the heating surface caused by the departure of bubbles whose space is replaced by the surrounding liquid body. However, its dynamic behavior at the same surface superheat differs when the boiling heat flux undergoes an increasing trend and when it undertakes a decreasing trend. The discrepancy in a boiling curve is generally recognized as boiling hysteresis which is especially evident in boiling of a highly wetting liquid. The present work investigates (i) difference in the start-up/restart cycles between the micro-configured graphite fiber-metal composite surface and pure metal surface, and (ii) mechanism for reduction of boiling thermal hysteresis on the composite surfaces. The graphite fiber surfaces possess two distinguished properties that are suitable to avoid retardation of bubble nucleation in highly wetting electronics cooling liquids. One is their ill-wetted character for most liquids and the other is their high thermal conductivity.