Bu-Xuan Wang

A fractal model for predicting the effective thermal conductivity of liquid with suspension of nanoparticles

Abstract Based on the effective medium approximation and the fractal theory for the description of nanoparticle cluster and its radial distribution, a method for modeling the effective thermal conductivity of “nanofluid” is established. The size effect and the surface adsorption of nanoparticles are taken into considerations. The proposed fractal model is discussed in detail for its application, and it predicts quite well with our recent measuring data for dilute suspensions of metallic oxide nanoparticles.

A novel concept for convective heat transfer enhancement

Abstract An analog between convection and conduction with heat sources is made to have a further understanding of the mechanism of convective heat transfer. There are three ways to raise the strength of heat sources/convection terms, and consequently to enhance the heat transfer: (a) increasing Reynolds and/or Prandtl number, (b) increasing the fullness of dimensionless velocity and/or temperature profiles, (c) increasing the included angle between the dimensionless velocity and temperature gradient vectors. Some approaches of heat transfer enhancement are suggested based on such a novel concept of heat transfer enhancement.

Experimental investigation on liquid forced-convection heat transfer through microchannels

Abstract Experiments were conducted to investigate the single-phase forced-flow convection of water or 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 by modifying the empirical constant coefficient from 0.023 to 0.00805. The calculated results are then in quite good agreement with experimental data. The transition and laminar heat transfer behavior in microchannels are very unusual and complex and are strongly affected by liquid temperature, velocity and microchannel size.

Forced convection and flow boiling heat transfer for liquid flowing through microchannels

Abstract Experiments were conducted to investigate the single-phase forced-flow convection and boiling characteristics of subcooled liquid flowing through microchannels with a cross-section of 0.6 × 0.7 mm, machined on the stainless steel plate 2 mm thick. The influences of liquid velocity and subcooling on the boiling curve were experimentally inspected. It was observed that a steep increase of q u emerged on the single-phase convection q u — T w curve. The experiments indicated that the single-phase convection and flow boiling characteristics are quite different from those in normally sized tubes, and their heat transfer was intensified. No apparent partial nucleate boiling exists for subcooled flow boiling, i.e. fully-developed boiling was induced much earlier in microchannels.

FRICTIONAL FLOW CHARACTERISTICS OF WATER FLOWING THROUGH RECTANGULAR MICROCHANNELS

Experiments were conducted to investigate the flow characteristics of water flowing through rectangular microchannels having hydraulic diameters of 0.133-0.367 mm and H/W ratios of 0.333-1. Experimental results indicated that the laminar flow transition occurred at Reynolds numbers of 200-700. This critical Re for the laminar transition was strongly affected by the hydraulic diameter, decreasing with corresponding decreases in the microchannel. In addition, the size of the transition range was diminished and fully developed turbulent flow also occurred at much lower Re. The friction behavior of both the laminar and turbulent flow was found to depart from the classical thermqfluid correlations. lite friction factor, f, was found to be proportional to Re−1.98 rather than Re for the laminar condition, and proportional to Re−1.72i for turbulent flow. The geometric parameters, hydraulic diameter, and H/W were found to be the most important parameters and had a critical effect on the flow. Generally, increasing...

Field synergy principle for enhancing convective heat transfer--its extension and numerical verifications

Abstract The concept of enhancing parabolic convective heat transfer by reducing the intersection angle between velocity and temperature gradient is reviewed and extended to elliptic fluid flow and heat transfer situation. Five examples of elliptic flow are provided to show the validity of the new concept (field synergy principle). Two further examples are supplemented to demonstrate the importance of the concept in the design of the enhanced surfaces.

Experimental investigation of heat transfer in flat plates with rectangular microchannels

Abstract An experimental investigation was conducted to determine the heat transfer characteristics and cooling performance of rectangular-shaped microgrooves machined into stainless steel plates. Using methanol as the cooling fluid, grooves with different aspect ratios and a variety of center-to-center spacings were evaluated. The influence of liquid velocity, subcooling, property variations and microchannel geometric configuration on the heat transfer behavior, cooling performance, and heat transfer and liquid flow mode transition were analyzed experimentally. Measurements made to clarify the flow nucleate boiling attributes indicated an increased heat transfer rate and a behavior that was quite different from what typically occurs in larger tubes or channels, due to the relatively large portion of the surface area associated with the thin-film region.

On the Specific Heat Capacity of CuO Nanofluid

This paper reviews briefly the definition of heat capacity and clarifies the defined specific heat capacity and volumetric heat capacity. The specific heat capacity and volumetric heat capacity, with our measured experimental data for CuO nanofluids, are discussed as an illustrating example. The result indicates that the specific heat capacity of CuO nanofluid decreases gradually with increasing volume concentration of nanoparticles. The measurement and the prediction from the thermal equilibrium model exhibit good agreement. The other simple mixing model fails to predict the specific heat capacity of CuO nanofluid. The nanoparticle size effect and solid-liquid interface effect on the specific heat capacity of nanofluid are discussed.

NUMERICAL SIMULATION OF FORCED CONVECTION HEAT TRANSFER IN POROUS PLATE CHANNELS USING THERMAL EQUILIBRIUM AND NONTHERMAL EQUILIBRIUM MODELS

A numerical study of fluid flow and convection heat transfer in a plate channel filled with metallic or nonmetallic particles using both the local thermal equilibrium model and a nonlocal thermal equilibrium model is presented in this paper. The numerical simulation results are compared with our experimental data, and a new modified thermal dispersion conductivity model is presented. The effects of the assumption of local thermal equilibrium versus nonlocal thermal equilibrium and the thermal dispersion effect on the convection heat transfer are investigated. The nonlocal thermal equilibrium model is appropriate for either nonmetallic or metallic porous media. The velocity distribution and the temperature fields are presented.

Jet flow phenomena during nucleate boiling

Abstract Boiling phenomena are with highly complex nonlinear and nonequilibrium characteristics, which cause diversity and complexity of boiling nucleation. In the present paper, an experimental investigation was conducted to investigate the nucleate boiling behavior on a very fine heating wire. Using zoom routine and CCD camera system, the dynamical process of nucleate boiling was visually observed and several modes of jet flows were explored during nucleate boiling. This phenomenon is quite different from the usual observation of nucleate boiling. High-energy liquid jet, fog-like jet, cluster-like jet, bubble-forming jet, bubble-bunch jet and bubble-top jet were described in detail. The microscopic mechanism concerning the phenomena was discussed.