Guodong Xia

Experimental and Numerical Study of Flow and Heat Transfer in Trapezoidal Microchannels

An experimental investigation has been performed on the laminar flow and heat transfer of water in trapezoidal silicon microchannels. Two three-dimensional (3D) heat transfer models have been developed to simulate the heat transfer performance under the same experimental conditions. Due to the sudden contraction, the velocity develops a little faster, which makes the pressure drop slightly lower than that of the neglected entrance and exit plenum regions. Due to the heat conduction in the lateral parts of wafer, the maximum temperature appears near the outlet of microchannel and the temperature is slightly different among the microchannels. Nearer to the lateral parts of wafer, the the temperature reaches the maximum for the sidewall later. With a given pumping power, the thermal resistance decreases as increase of the heating power at the substrate. However, the subthermal resistance proportion is nearly unchanged. With an increase of pumping power, the subthermal resistance proportion of convection increases rapidly at first, then gradually approaches an asymptote.

Numerical simulation and multiobjective optimization of a microchannel heat sink with arc-shaped grooves and ribs

ABSTRACT In order to obtain the optimal structure size of a microchannel heat sink (MCHS) with arc-shaped grooves and ribs according to the actual demand, multiple parameters that influence the performance of the microchannel are analyzed by combining the multi-objective evolutionary algorithm (MOEA) with computational fluid dynamics (CFD). The design variables include the relative groove height, relative rib height and relative rib width, and the two objective functions are to minimize the total thermal resistance and pumping power in constant volume flow rate. The influences of the design variables on the two objective functions are analyzed by CFD firstly. The results show that each design variable has a different impact on the two functions. The competitive relationship between the two objective functions is depicted in plots of the Pareto front obtained by MOEA. Pareto sensitivity analysis is carried out to find that the relative rib height has the most significant impact on the two objective functions.

Single-phase and two-phase flows through helical rectangular channels in single screw expander prototype

The CFD simulations are carried out for the flows in a horizontally oriented helical pipe with various inlet sectional liquid holdups and coil pitches (H). The development of the pressure fields for the single phase air flow and the air-water two-phase flow through the helical rectangular channels is studied. The points with a higher pressure often become the position of expansion leakage. The liquid phase distribution at these points can prevent the leakage of air. It is shown that the increase of the inlet sectional liquid holdup may increase the local liquid holdup at the outmost side of the helical channel. Based on the published pressure drop correlation, a new modified relation for predicting the pressure drop in the helical rectangle channel is proposed.

Flow Resistance Characteristics of De‐ionized Water Flow through Staggered Diamond and Circular Micro Pin Fin Arrays

This paper reports an experimental investigation on flow resistance characteristics associated with the forced flow of de‐ionized water through circular and diamond shaped micro pin fin arrays. Comparing with circular pin fin arrays, diamond shaped pins produce higher friction factors. By comparing the current data with results available in the literature, it can be concluded that the conventional long tube correlations are unable to predict the friction factor for micro pin fin arrays. According to the experimental data, several new correlations for friction factor and Reynolds number of staggered pin fin arrays were developed.

Fundamental Issues, Technology Development, and Challenges of Boiling Heat Transfer, Critical Heat Flux, and Two-Phase Flow Phenomena with Nanofluids

ABSTRACT This paper presents a comprehensive and critical review of studies on nucleate pool boiling heat transfer, flow boiling heat transfer, critical heat flux (CHF), and two-phase flow phenomena with nanofluids. First, general analysis of the available studies on the relevant topics is presented. Then, studies of physical properties of nanofluids are discussed. Next, boiling heat transfer, CHF phenomena, and the relevant physical mechanisms are explored. Finally, future research needs have been identified according to the review and analysis. As the first priority, the physical properties of nanofluids have a significant effect on the boiling and CHF characteristics but the lack of the accurate knowledge of the physical properties has greatly limited the studies. Fundamentals of boiling heat transfer and CHF phenomena with nanofluids have not yet been well understood. Flow regimes are important in understanding the boiling and CHF phenomena and should be focused on. Two-phase pressure drops of nanofluids should also be studied. Furthermore, economic evaluation of the enhancement technology with nanofluid should be considered for the new heat transfer enhancement technology with nanofluids. Finally, applied research should be targeted to achieve an enabling practical heat transfer and CHF enhancement technology for engineering application with nanofluids.

Heat transfer characteristics and flow visualization during flow boiling of acetone in semi-open multi-microchannels

ABSTRACT Experimental results of flow boiling characteristics and flow patterns with acetone in two different microchannel heat sinks are presented in this paper. A semi-open microchannel heat sink and a straight microchannel heat sink with 19 parallel microchannels each were designed and tested. The semi-open microchannels have a channel width of 0.8 mm, fin width of 0.4 mm, and pedestal height of 0.2 mm and the straight microchannels have a rectangular cross section of 0.8 mm × 1 mm. The experimental heat fluxes ranged from 0 to 90 kW/m2, vapor quality ranged from 0.05 to 0.5, mass fluxes ranged from 4.34 to 15.62 kg/m2·s and the inlet temperatures were 20 and 30°C, respectively. Compared to those in the straight microchannels, flow boiling heat transfer coefficients can be improved by up to 36.2%. Furthermore, flow patterns were observed with a speed video camera. The flow boiling heat transfer mechanisms are analyzed according to the observed flow patterns.

Effect of geometric configuration on the laminar flow and heat transfer in microchannel heat sinks with cavities and fins

ABSTRACT A numerical simulation is performed to investigate the characteristics of flow and heat transfer in microchannels with cavities and fins. Nine microchannels with various shaped cavities and fins are presented and compared to the smooth microchannel. The effect of cavity and fin shapes on the flow field and temperature field is analyzed. Results show that the presence of cavity and fin can increase the heat transfer area, intensify mainstream disturbance, and induce chaotic advection, which result in obvious heat transfer enhancement. The shape of cavity or fin has a great influence on the hydrodynamic and thermal performance for such micro heat sinks. Based on the performance evaluation criterion (PEC), the overall performance of the microchannel is evaluated. The combination of cavities and fins leads to lower bottom temperature, lower net temperature gradient of fluid, and better heat transfer performance, which has the potential to meet the increased heat removal requirement.

Effects of structural parameters on fluid flow and mixing performance in a curved microchannel with gaps and baffles

We provide three-dimensional numerical simulations of mixing performance in a newly proposed micromixer with different structural parameters. The same amount of gaps and baffles are arranged along the curved channel within a certain distance. The effects of their structural parameters on mixing efficiency are presented, which include either the position and feature size of gaps and baffles, or the curvature radius of curved channel. The high efficiency mixing mechanism of the curved channel with gaps and baffles can attribute to the interaction of the increased contact area for premixed liquids, the jet and throttling effect over every unit of gap and baffle, the developing of the multidirectional vortices along the curved channel. The mixing index is sensitive to the width of the gaps and baffles for some Reynolds number ranges, but is not sensitive to the curvature radius of the curved channel. The characteristic of the pressure drop depending on Reynolds number is also investigated in order to keep an ...

The heat transfer characteristics of the jacket-type radial heat pipe

A simple, rapid mathematical model to calculate the non-steady-state startup process of the jacket-type radial heat pipe is presented in this paper. The model is based on the special structure and using conditions of the jacket-type radial heat pipe, the vapor temperature in heat pipe only changes over time. The startup performance of the heat pipe with variation input heat flux and the filling rate is analyzed. The results manifest that the filling rate increased will reduce the maximum operating temperature of the heat pipe and shorten the startup time of the heat pipe. With the increase of input heat flux, the operating temperature increases and the time to reach the steady state of the heat pipe is added. The total thermal resistance of heat pipe decreases with the increase of the input heat flux and filling rate. The variation of the local convective heat transfer coefficient and the maximum temperature of the water are investigated with different cooling water inlet conditions.

A Novel Passive Micromixer Based on Asymmetric Split-and-Recombine With Fan-Shaped Cavity

A novel passive micromixer with fan-shaped cavity based on the principle of flow planar asymmetric split-and-recombine (P-ASAR) and focusing/diverging is designed. The micromixer consists of two split sub-channels with unequal widths and one fan-shaped cavity structure on the major sub-channel which are similar to a diamond ring structure. In order to yield optimum mixing effect, different parameters of geometry structure under a wide range of Reynolds numbers (1–80) have been investigated by numerical simulation with three-dimensional Navier-Stokes equations. The steady laminar flow was solved by using a finite-volume method and SIMPLE algorithm. Enhanced micromixing is achieved by utilizing a synergistic combination of unbalanced inertial collision, Dean vortices and expansion vortices. As a result of interplay between inertial, centrifugal and viscous effects, Dean vortices arise in the vertical plane of curved channel. Expansion vortices appear in the horizontal plane due to an abrupt increase of the cross-sectional area. The mixing index is used to evaluate the degree of mixing. Our studies show that vortices are observed in the channels at high Reynolds numbers. The geometry parameters of fan-shaped cavity structure affect the mixing index of micromixer. When the ratio of the widths of the major sub-channel and fan-shaped cavity channel is 1/3, the mixing index of this type micromixer could reach around 75% at Reynolds numbers larger than 60. The relation between mixing intensity and pump power consumption has been analyzed at a wide range of Reynolds numbers simultaneously.Copyright