Research on the Heat-Conduction Enhancement for Liquid with Nano-Particle Suspensions

By measuring the effective thermal conductivity, taking photos of the distribution of the nano-particles in the fluids and photos that reflect the interfacial phenomena between nano-particles and fluids, we try to explain the possible mechanism for heat conductive enhancement.

Dynamic Behavior of Bubble Interface During Boiling

A brief review with discussions is conducted for some pertinent works, done and ongoing in the Laboratory of Phase-Change and Interfacial Phenomena at Tsinghua University, on interfacial behavior of vapor bubbles and interfacial transport phenomena during liquid nucleation boiling. From a sequence of experimental investigations, some new phenomena, particularly, the visually observed interfacial transport phenomena or processes including jet-like flows, bubble interaction and spatial scale effect, were described in this article. The interfacial effects and transport phenomena associated with surface tension gradients caused by temperature and concentration variations were theoretically analyzed to reveal the marked influence on bubble interfacial shape and dynamic behavior, the bubble dynamics including nucleation, bubble motion and coalescence. Several theoretical models and methods were proposed to describe the dynamic characteristics and explain the physics of interfacial phenomena/processes. The

Cooling Augmentation with Microchanneled Structures

Experiments were conducted to investigate the heat transfer characteristics and cooling performance of subcooled liquid, water, flowing through rectangular cross-section microchanneled structures machined on a stainless steel plate. Heat transfer or flow mode transition was observed when the heating rate or wall temperature was increased. This transition was found to be suggestively induced by the variation in liquid thermophysical properties due to the significant rise of liquid temperature in the microstructures. The influence of such parameters as liquid velocity, subcooling, property variation, and microchannel geometric configuration on the heat transfer behavior, cooling performance and the heat transfer and liquid flow mode transition were also investigated. The experiments indicated that both slngle-phase forced convection and flow boiling characteristics were quite different from those in normal-sized tubes and the heat transfer was obviously intensified.

Interface Shape and Marangoni Effect Around aBubble Within the Thermal Boundary Layer

The characteristics of a vapor bubble within the thermal boundary layer were theoretically analyzed.The physical models accounting for the variation of ioterfacial tension and nuid density with tempera-ture were proposed to investigate bubble interfaCe aspects and the fluid flow around the bubble. The analyses demonstrated that the variation in interfacial tension results in variations in the liquid-vapor interface shape and bubble dynamics, which may play a significant role in the departure process of a vapor bubble from a heated wall surface. Increasing temperature gradients in the boundary layer and the gravitational field induce a contact line contraction and correspondingly promotes bubble depar-ture. The simulation of liquid now around the bubble shows that natural convection dominates the flow for earth conditions; however, the thermocapillary forces provide the principal catalyst for bubble departure in a microgravity environment. The results indicate that both the vapor bubble contraction and the Marangoni flow may increase the heat transfer around the vapor bubble and may cause the bubble to mov away from the heating surface, further increasing heat transfer.

Forced-Flow Convection for Liquid Methanol Flowing through Microchannels

Experiments were conducted to investigate the single phase forced-flow convection of methanol flowing through microchannels with rectangular cross-section.The fully-developed turbulent convection regime was found to be initiated at about Re=1000-1500,The fully developed turbulent heat transfer can be predicted by the well-known Dittus-Boelter correlation with mere modification of the original empirical constant coefficient 0.023 to 0.00805.The transition and laminar heat transfer behaviors in microchannels are highly peculiar and complicated,and heavily affected by liquid temperature,velocity and microchannel size.

Relative Stability of Boiling of FC-72 and HFE-7100 with Applications toElectronic Device Cooling

This paper investigates the relative stability between nucleate and film boiling modes of FC-72 and HFE-7100, which have potential to electronic device cooling applications. Equilibrium heat flux, qc, which refers to as an index for measuring the relative stability of boiling, was obtained at a liquid subcooling of 0-20 K. Experimental results reveal that (1) qc increases with liquid subcooling; (2) although the FC-72 exhibits a higher critical heat flux (CHF) than does the HFE-7100, somewhat unexpectedly, the equilibrium heat flux for the latter is greater than the former. Restated, at a prescribed heat flux, the risk to burnout for boiling of FC-72 is higher than that of HFE-7100. The shift in boiling curves interprets the experimental findings.