YanFeng Fan

Mixing Evaluation of a Passive Scaled-Up Serpentine Micromixer With Slanted Grooves

A novel, passive, scaled-up micromixer based on fluid rotation is proposed and evaluated experimentally and numerically over Reynolds numbers ranging from 0.5 to 100. Flow visualization is employed to qualitatively assess flow patterns, while induced fluorescence is used to quantify species distribution at five locations along the channel length. Two individual fluids are supplied to the test section via a Y-inlet. The fluid enters a meandering channel with four semicircular portions, each of which is lined with nine slanted grooves at the bottom surface. The main mixing channel is 3 mm wide and 0.75 mm deep, with a total length of 155.8 mm. Numerical simulations confirm rotation at all investigated Reynolds numbers, and the strength of rotation increases with increasing Reynolds number. Grooves are employed to promote helical flow, while the serpentine channel structure results in the formation of Dean vortices at Re ≥ 50 (Dean number ≥ 18.25), where momentum has a more significant effect. A decreasing-increasing trend in the degree of mixing was noted, with an inflection point at Re = 5, marking the transition from diffusion dominance to advection dominance. The increase in interfacial surface area is credited with the improved mixing in the advection-dominant regime, while high residence time allowed for significant mass diffusion in the diffusion-dominant regime. Good mixing was achieved at both high and low Reynolds numbers, with a maximum mixing index of 0.90 at Re = 100.

Experimental Investigation of Flow Boiling Instability in a Single Horizontal Microtube With and Without Inlet Restriction

An experimental study is conducted to investigate the effects of inlet restriction (orifice) on flow boiling instability in a single horizontal microtube. The test-section is composed of a stainless steel tube with an inner diameter of 889 μm, and a length of 150 mm. Experiments are performed for three different orifice configurations with 20%, 35%, and 50% area ratio. Mass flux is varied from 700 to 3000 kg/m 2 · s, whereas the heat flux is varied from 6 to 27 W/cm 2 . The dielectric coolant FC-72 is selected as the working fluid. In the absence of an orifice at the inlet, four oscillation types are observed at the onset of flow instability; it is also noticed that the frequency of the oscillations increases with increasing heat flux, while the amplitude remains constant. The addition of an orifice at the inlet helps stabilizing the flow without generating significant pressure drop at the same operating condition as the microtcrbe without orifice. The 20% area ratio orifice shows better performance at low mass fluxes ( 2000 kg/m 2 · s), 50% and 35% area ratio orifices are efficient in stabilizing the flow or delaying the onset of flow instability. Therefore, selecting the area ratio of the orifice depends on the operating condition. A small area ratio orifice is preferably used at low mass fluxes, whereas a large area ratio orifice is more suitable for high mass fluxes.

Effect of Inlet Restriction on Flow Boiling Heat Transfer in a Horizontal Microtube

Flow boiling heat transfer in a horizontal microtube with inlet restriction (orifice) under uniform heating condition is experimentally investigated using FC-72 as working fluid. A stainless steel microtube with an inner diameter of 889 μm is selected as main microtube. Two microtubes with smaller diameters are assembled at the inlet of main microtube to achieve the restriction ratios of 50% and 20%. The experimental measurement is carried out at mass fluxes ranging from 160 to 870 kg/m2·s, heat fluxes varying from 6 to 170 kW/m2, inlet temperatures of 23 and 35 °C, and saturation pressures of 10 and 45 kPa. The effects of the orifices on two-phase pressure drop, critical heat flux (CHF), and flow boiling heat transfer coefficient are studied. The results show that the pressure drop caused by the orifice takes a considerable portion in the total pressure drop at low mass fluxes. This ratio decreases as the vapor quality or mass flux increases. The difference of normal critical heat flux in the microtubes with different orifice sizes is negligible. In the aspect of flow boiling heat transfer, the orifice is able to enhance the heat transfer at low mass flux and high saturation pressure, which indicates the contribution of orifice in the nucleate boiling dominated regime. However, the effect of orifice on flow boiling heat transfer is negligible in the forced convective boiling dominated regime.

Prediction of the Onset of Flow Instability in a Single Horizontal Microtube With an Inlet Orifice

A methodology to predict the onset of flow instability (OFI) in a single horizontal microtube with an inlet orifice is developed based on the predication of pressure drop. The predictive methodology states, for the same flow rate, the flow instability occurs as the single-phase liquid pressure drop under no heating condition equals the two-phase pressure drop under heating condition in a single microtube. The addition of inlet orifice increases the heat flux at the onset of flow instability by increasing the upstream pressure. The present methodology is validated by comparing the predicted heat flux at the onset of flow instability with our previous experimental data in the microtubes with three sizes of inlet orifices. The results show that the present method can predict the heat flux at the onset of flow instability with a deviation of 30% and mean absolute error of 13% at mass fluxes from 700 to 3000 kg/m2 s. The effects of inlet orifice size and saturation pressure on the onset of flow instability are also studied based on the present methodology. It is found that, at mass fluxes from 100 to 2000 kg/m2 s, the area ratio less than 15% eliminates the flow instability completely before the critical heat flux occurs.

The Effect of Inlet Orifice on Critical Heat Flux and Flow Boiling Heat Transfer in Single Horizontal Microtube

Flow oscillation is a crucial issue for the development of flow boiling heat transfer in the applications. Inlet orifice has been proven be an option to eliminate the oscillation. However, the effects of inlet orifice on critical heat flux and flow boiling heat transfer coefficient are lack of study. In this work, the effects of inlet restriction on critical heat flux and heat transfer coefficient in single horizontal microtube under uniform heating condition is experimentally investigated using FC-72 as working fluid. A stainless steel microtube with an inner diameter of 889 μm is selected as main microtube. Two smaller microtubes are assembled at the inlet of main microtube to achieve the restriction configurations of 50% and 20% area ratios. The experimental measurement is carried out at mass fluxes ranging from 160–870 kg/m2·s and heat fluxes varying from 6–170 kW/m2. Two saturation pressures, 10 and 45 kPa, are tested. The experimental results of critical heat flux and two phase heat transfer coefficient obtained in the microtube without orifice are compared with the existing correlations. The addition of an orifice does not enhance the normal critical heat flux but increases the premature critical heat flux. In aspect of heat transfer, the orifice shows improvement on heat transfer coefficient at low mass flux and high saturation pressure.Copyright

Numerical Simulation of a Novel Jet-Impingement Micro Heat Sink With Cross Flow Under Non Uniform Heating Condition

A novel micro heat sink applying the jet-impingement and cross flow is proposed to dissipate the heat from the electrical devices. Six hotspots of 2 mm × 2 mm are positioned on a flat plate of 25.4 mm × 25.4 mm. The area of flat plate except the hotspots is provided a constant heat flux of 20 W/cm2 as background heating source among cases. Four heat fluxes from 40 to 100 W/cm2 on the hotspots are tested to simulate the different operation conditions. The cross flow is used to remove the background heat flux and jet flow is supplied into the swirl microchannel, located at the right top of hotspot, to dissipate the large heat flux from hotspots. The channel depth is 0.5 mm and the width of swirl microchannel is 0.38 mm. The cross flow and jet flow velocity vary from 0.1 m/s to 0.5 m/s and from 0.5 m/s to 2 m/s, respectively. The effects of cross flow and jet flow on the cooling performance are investigated by numerical simulation. The local heat transfer coefficient and Nusselt number are calculated to evaluate the cooling performance of proposed micro heat sink for the targets of low maximum temperature, temperature gradient and pressure drop. The results show that the maximum temperature of the proposed design occurred at the outlet is approximately 65 °C among tested cases. The corresponding pressure drop is 5.5 kPa. The overall thermal resistance reaches as small as 0.23 K/W.Copyright

New Predictive Methodology for the Onset of Flow Instability in Single Horizontal Microtube With an Inlet Orifice

A new methodology to predict the onset of flow instability (OFI) in single horizontal microtube with inlet orifice is proposed. The predictive methodology states that OFI occurs as the pumping power under no heating condition is equal to the pumping power under heating condition in the microtube at the same volume rate. Since the pumping power can be simply described as the product of volume rate and pressure drop cross the microtube, the heat flux at OFI is determined as the two-phase pressure drop under heating condition is equal to the single-phase pressure drop under no heating condition at same flow rate. The addition of inlet orifice increases the pumping power under no heating condition. The increased pumping power by orifice delays the onset of flow instability. The predictive methodology is validated by comparing the predicted heat flux at OFI with our previous experimental data in the microtubes with three different inlet restriction ratios. The result shows that the proposed method is capable of prediction of heat flux at OFI with a deviation of 30% and mean absolute error of 13% at mass flux less than 2000 kg/m2·s.Copyright

Experimental Investigation of a Scaled-Up Passive Interdigital Micromixer With Circular-Sector Mixing Elements

Flow patterns and mixing phenomena are investigated qualitatively in a planar passive scaled-up micromixer using flow visualization over 5 ≤ Re ≤ 200. To promote molecular diffusion, the test section utilizes an uneven interdigital inlet which reduces the diffusion path and enhances mixing at the side walls. Five circular sector obstructions located along the channel length serve to divide and recombine the flow, as well as induce Dean vortex formation at high Reynolds numbers. Induced fluorescence is used to provide a quantitative estimate of mixing efficiency at certain Reynolds numbers. A decreasing-increasing trend in mixing efficiency is observed with increasing Reynolds numbers, marking the transition from mass diffusion dominance to mass advection dominance. The design operates well at higher Reynolds numbers, where the dominant mixing mechanism is mass advection.Copyright

Numerical Study of Microchannel Heat Sinks With Non-Uniform Heat Flux Conditions

Higher heat flux is produced by Micro-Electro-Mechanical Systems (MEMS) because of their reduced size and increased clock speed. At the mean time, studies of non-uniform heating conditions which are more practical than uniform heating conditions are inadequate and needed urgently. Four nonuniform heating conditions are simulated in the paper. Three heat sinks with different widths of cross-linked channels locating above the center of hotspots are studied and compared to conventional straight microchannel heat sink. Half of the module geometry is chosen to be the computational domain. Two hotspots are placed at the bottom surface. The coolant is water, whose properties are dependent on temperature. Two inlet velocities, 0.5 m/s and 1 m/s, are tested for each heat sink. Temperature profile at the hotspots, pressure drop and total thermal resistance are selected as criteria of evaluating heat sink performance. All heat sinks have better performance when there is an upstream hotspot or the upstream hotspot is subjected to a higher heat flux. Cross-linked channel width of 0.5 mm has the best benefit to obtain better temperature uniformity without increasing the maximum temperature on the bottom surface.Copyright

Investigation of Cooling Performance of a Swirl Microchannel Heat Sink by Numerical Simulation

High heat fluxes have been created by the semiconductor devices due to the high power generation and shrank size. The large heat flux causes the circuit to exceed its allowable temperature and may experience both working efficiency loss and irreversible damage due to excess in their temperatures. In this paper, a swirl microchannel heat sink is designed to dissipate the large heat flux from the devices. The numerical simulation is carried out to investigate the cooling performance. Uniform heating boundary condition is applied and single phase water is selected as coolant. The present micro heat sink applies multiple swirl microchannels positioned in a circular flat plate to enhance the heat convection by creating the secondary flow at high Reynolds numbers. Copper is selected as the material of heat sink. The channel depth and width are fixed as 0.5 mm and 0.4 mm, respectively. The heat is injected into the system from the bottom of heat sink at the heat fluxes from 10 to 60 W/cm2 . Flow is supplied from the top of micro heat sink through a jet hole with a diameter of 2 mm and enters swirl microchannels at the volume flow rates varying from 47 to 188 ml/min. The cooling performances of swirl microchannel heat sinks with different curvatures and channel numbers are evaluated based on the targets of low maximum temperature, temperature gradient and pressure drop.Copyright