An Investigation On Single-And Two-Phase Pressure Losses In Mill-And Microchannels

Due to the widespread applications of microchannels in MEMS, microelectronics, bioengineering and biotechnology, communication, specially in aerospace,the study of characteristics of microscale momentum and heat transfer becomes an important subject of recent investigation. Recently twenty years, a large number of research results show the characteristics of momentum and heat transfer in microchannels are different from those in conventional one and with scale effect, and the conclusions and the results have obvious confusions or contradictions each other. The main purpose of the present study is to clarify some mechanisms about fluid flowing through microchannels. The experimental study and theoretical analysis on single- and two-phase flow in microchannels is performed in this work with emphasis put on the minor pressure drop in microchannels.Firstly, a novel pressure measurement technique, the tiny gaps on the tubes, was developed. In order to verify the feasibility of pressure measurement though a gap, the experiments of water and nitrogen flow through the stainless microtube with inner diameters 336μm were performed. The relationship between friction factor and Reynolds number is obtained by measuring the pressure drop and the flow rate. The experimental results show the pressure measurement through gaps for microtubes is a feasible method. The error of pressure measurement is very small for the ratio of gap width and tube diameterξ< 0.2 and doesn't change with the increasing of fluid velocity, so the width of gap should be smaller than 0.2D.Secondly, this thesis investigated single-phase and gas-liquid two-phase flow across the abrupt expansion and contraction in microtubes with the diameter from 330μm to 850μm, using de-ionized water, toluene, ethanol and nitrogen at room temperature and atmospheric as the working fluids. The experimental results on pressure drop with measurement though gaps were used to characterize the minor losses in microtubes. The ranges of single-phase flow Reynolds numbers and mass quality in two-phase flow were 589～8520 and 2.6×10^-3～1.63×10^-1 in the smaller tube respectively. In single-phase flow experiments, the expansion loss coefficients were slightly larger than the experimental results from conventional tubes in the laminar flow; while in the turbulent flow, the expansion loss coefficients were roughly consistent with those from conventional tubes with exception of expansion loss coefficients for nitrogen showing a linear change with Reynolds number in the smaller tube, the reason may be that the local Mach number is more than 0.3, the effect of compressibility should be fully considered, so the data deduction method for flow area change in conventional tube doesn't suitable for microtubes. The contraction loss coefficients were larger than those from the conventional tubes in the laminar flow; while in the turbulent flow, the contraction loss coefficients were slightly smaller than those from conventional tubes and predicted well by Kc = 0.5×(1-σ)^0.75. In two-phase flow experiments, the two-phase flow pressure drops caused by expansion were significantly lower than the homogeneous flow model and those by contraction significantly higher than the homogeneous flow model; the slip flow model with a velocity slip ratio S = (ρ_L/ρ_G)^1/3 showed a good prediction that reveals the occurrence of velocity slip. An empirical correlation for two-phase flow pressure drops caused by the sudden contraction was developed based on the proposed contraction loss coefficients correlation for single-phase flow and Lockhart-Martinelli correlation. Thirdly, microtubes are commonly characterized by higher relatively roughness.Computions are carried out to investigate the effect of two-dimensional roughness elements on the sudden contraction pressure drop in microtubes and a preliminary theoretical model is brought forward. It can be concluded that higher pressure loss for incompressible fluid flowing through sudden contraction in rough microtubes can be partly attributed to the roughness elements.Finally, by the means of combination of high-speed camera and pressure signal fluctuation, two-phase frictional pressure drop and flow regimes in small horizontal rectangular channels with 0.99mm inner diameter were experimentally investigated, using nitrogen and water. The flow regime map and the characteristics of frictional pressure drop in different flow regimes were obtained. Comparisons between the experimental data and some predictions indicate that the L-M model shows a better predictive ability than the other empirical correlations.