Full field measurement at the micro-scale using micro-interferometry

This paper presents micro-interferometry as a measurement technique to extract temperature profiles and/or mass transfer gradients rapidly and locally in micro-devices. Interferometry quantifies the phase change between two or more coherent light beams induced by temperature and/or mass concentration. Previous work has shown that temporal noise is a limiting factor in microscale applications. This paper examines phase stepping and heterodyne phase retrieval techniques with both CCD and CMOS cameras. CMOS cameras are examined owing to the high speed at which images can be acquired which is particularly relevant to heterodyne methods. It is found that heterodyne retrieval is five times better than phase stepping being limited to 0.01 rad or λ/628. This is twice the theoretical limit of λ/1,000. The technique is demonstrated for mixing in a T-junction with a 500 μm square channel and compared favourably to a theoretical prediction from the literature. Further issues regarding application to temperature measurements are discussed.     

Charge reduction experimental investigation of CO2 single-phase flow in a horizontal micro-channel with constant heat flux conditions

The use of carbon dioxide as alternative refrigerant in refrigeration plants and heat pumps has been focused recently. Through the specific properties of CO₂, the use of very compact heat exchangers is relevant and the technology of micro-channel heat exchangers rises as a suitable solution. The experimental investigation of CO₂ flow in a single micro-channel with an inner diameter of 529 μm is planned with an original test section. This test section is initially dedicated for further CO₂ two-phase flow analysis. The local heat transfer coefficients are estimated with micro-thermocouples stuck on the micro-channel wall. The pressure drop is also measured. This paper presents the first results in single-phase pressure drop and heat transfer and exhibits promising coming data in two-phase flow pressure drop and heat transfer for mass velocity between 200 kg/m²/s and 1400 kg/m²/s and working saturation temperature between −10 °C and 5 °C. The results stress on the good accuracy of suitable classical laws to predict pressure drop and heat transfer in single-phase flow in micro-channel.     

Transmission properties of the single mode fiber with a cross-sectional micro-channel investigated using time-domain finite difference (FDTD) method

Light scattering by a micro-channel intersecting core of a single mode fiber (SMF) has been investigated by use of two dimensional time-domain finite difference (2D-FDTD) method and the numerical results are in good agreement with those reported in the experiments. The transmission properties of the SMF with such a micro-channel can be attributed to the combined results of scattering effect and FP resonance effect. It is suggested that the SMF with micro-channel of about 4–6 μm in diameter can be effectively used for refractive index sensing with a high refractive index resolution up to ∼10⁻⁵.     

Heat transfer in micro-channels: Comparison of experiments with theory and numerical results

This paper considers experimental and theoretical investigations on single-phase heat transfer in micro-channels. It is the second part of general exploration “Flow and heat transfer in micro-channels”. The first part discussed several aspects of flow in micro-channels, as pressure drop, transition from laminar to turbulent flow, etc. [G. Hetsroni, A. Mosyak, E. Pogrebnyak, L.P. Yarin, Fluid flow in micro-channels, Int. J. Heat Mass Transfer 48 (2005) 1982–1998]. In this paper, the problem of heat transfer is considered in the frame of a continuum model, corresponding to small Knudsen number. The data of heat transfer in circular, triangular, rectangular, and trapezoidal micro-channels with hydraulic diameters ranging from 60 μm to 2000 μm are analyzed. The effects of geometry, axial heat flux due to thermal conduction through the working fluid and channel walls, as well as the energy dissipation are discussed. We focus on comparing experimental data, obtained by number of investigators, to conventional theory on heat transfer. The analysis was performed on possible sources of unexpected effects reported in some experimental investigations.     

Application of a micro-channel reactor for process intensification in high purity syngas production via H2O/CO2 co-splitting

A stainless steel micro-channel reactor was tailor-made to an in house-design for process intensification propose. The reactor was used for a two-step thermochemical cycles of H₂O and CO₂ co-splitting reaction, in the presence of La_0.3Sr_0.7Co_0.7Fe_0.3O₃ (LSCF). LSCF was coated inside the reactor using wash-coat technique. Oxygen storage capacity of LSCF was determined at 4465 μmol/g, using H₂-TPR technique. H₂O-TPSR and CO₂-TPSR results suggested that a formation of surface hydroxyl group was the cause of H₂O splitting favorable behavior of LSCF. Optimal operating reduction/oxidation temperature was found at 700 °C, giving 2266 μmol/g of H₂, 705 μmol/g of CO, and 67% of solid conversion, when the H₂O and CO₂ ratio was 1 to 1, and WSHV was 186,000 mL/g.h. Activation energy of H₂O spitting and CO₂ splitting was estimated at 87.33 kJ/mol, and 102.85 kJ/mol The pre-exponential factor of H2O splitting and CO2 splitting was 595.24 s^?1 and 698.79 s^?1, respectively.     

An experimental investigation of refrigerant mixture R32/R290 as drop-in replacement for HFC410A in household air conditioners

In the present paper, the refrigerant mixture R32/R290 (68%/32% by weight) is investigated as the drop-in replacement for R410A in household air conditioners. The GWP of it is only 22% of that of R410A. Theoretical and experimental investigations are conducted on the performance of the air conditioners working with both R32/R290 and R410A. Experimental results show that the refrigerant charge amount of R32/R290 is reduced by 30.0%–35.0%; the cooling and heating capacities are increased by 14.0%–23.7%. For further reducing charge amount and flammability, the micro-channel heat exchanger (condenser) is employed to replace the finned tube one. Compared with the R32/R290 system using finned tube heat exchanger, the R32/R290 charge amount and the power consumption are reduced by 34.1% and 0.4%, respectively; the cooling capacity and the COP are increased by 6.4% and 6.8%, respectively.     

Two-stage forming approach for manufacturing ferritic stainless steel bipolar plates in PEM fuel cell: Experiments and numerical simulations

Multi-stage micro-channel forming by stamping, as a method for cost effective and efficient for mass production, was performed for ultra-thin ferritic stainless steel sheets with thicknesses of 0.1 and 0.075 mm, as a good substitute for traditional graphite bipolar plates of proton exchange membrane fuel cell. Attention was directed to enhance the final forming depth and minimize localized thinning, extremely important aspects of the micro-channel on bipolar plate, by the proposed forming process. A forming depth at the first forming stage was chosen as a process variable, and its effect on the formability of the micro-channel at the second forming stage was experimentally investigated. Finite element simulations for the two-stage forming process were conducted to optimize the punch radius and forming depth at the first stage for improving the formability. The comparative study between the simulations and the experimental results could validate improvements in the formability by the proposed approach. In particular, this study could support the existence of an optimum forming depth at the first forming stage. Based on the simulation results, a mathematical model was established to identify the dominant factor needed for formability improvement and to propose a methodology for the process optimization of the multi-stage forming.     

The effect of wall roughness on the liquid removal in micro-channels related to a proton exchange membrane fuel cell(PEMFC)

In this paper, the effect of the wall roughness on the water behavior related to the PEMFCs gas channel is investigated by the two-phase flow simulation. And, the different wetting conditions of the wall surface are considered, i.e. hydrophilic surface and hydrophobic surface. The relative roughness height and the roughness element density as well as the roughness element type are also considered in the study. And the results show: (1) for hydrophilic surface, water behavior for smooth case is different from the roughness cases, due to the effect of roughness on the water slug morphology even for r/H = 0.2% roughness. (2) r/H = 0.2% is positive for water removal and will not lead to the high pressure drop for hydrophilic surface, (3) r/H = 5% is advantageous for water removal for hydrophilic surface but disadvantageous for hydrophobic case, and the pressure drop greatly increases for both cases, (4) for hydrophobic surface, roughness of r/H = 1% and r/H = 2% slow down the water removal speed, but will not affect the amount of the removable water, (5) there is nearly no effect for r/H = 0.2% for hydrophobic case, (6) for both conditions, the average pressure drop obviously increases when r/H ≥ 2%. (7) Increase of the roughness element can help water removal for hydrophilic case but no obvious function for hydrophobic surface. (8) The triangle roughness element is better than rectangle element with the same height.     

Evaluation of a micro-channel reactor for steam reforming of ethylene glycol: A comparative study of catalytic activity of Pt or/and Ni supported γ-alumina catalysts

Steam reforming of ethylene glycol (EG) was studied using γ-alumina supported 12%Ni, 3%Pt and 3%Pt12%Ni catalysts, in a micro-channel reactor. The parallel micro-channels were etched on a stainless steel plate using micro-milling technique with high speed CNC machine. The catalysts were prepared by the incipient wetness impregnation method and were characterized by using XRD, BET, FE-SEM, H₂-TPR and TGA analyses. The effects of reaction temperature and feed flow rate on the EG conversion, hydrogen yield and selectivities of the gaseous products were investigated. Experimental findings revealed that 3%Pt12%Ni/γ-alumina catalyst can provide the highest EG conversion (96.1%) with 76.6% hydrogen yield and 5.3% CO selectivity at 450 °C temperature and 4 mL h^?1 feed flow rate. Furthermore, continuous EG steam reforming identified 3%Pt12%Ni/γ-alumina as the most stable catalyst. This catalyst can remain stable after being on stream for more than 20 h.