Total temperature measurements of laminar gas flow at micro-tube outlet: Cooled from the wall

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.This paper presents experimental results on heat transfer characteristics of laminar gas flow in a micro-tube with constant wall temperature whose wall temperature is lower than the inlet temperature (cooled case). The experiment was performed for nitrogen gas flow through micro-tubes with 163 and 243 μm in diameter and 50 mm in length. The gas was heated in an upstream section of the micro-tube to Tin=315K, 335K and 355K. The wall temperature was maintained at 305K by circulating water around the micro-tube. The stagnation pressure was chosen in such a way that the exit Mach number ranges from 0.1 to 0.7. The outlet pressure was fixed at the atmospheric condition. The total temperature at the outlet, the inlet stagnation temperature, the mass flow rate and the inlet temperature were measured. The numerical computations based on the Arbitrary – Lagrangian – Eulerian (ALE) method were also performed for the same conditions of the experiment for validation of numerical results. The both results are in excellent agreement. The total and bulk temperatures obtained by the present study are also compared with the temperature of the incompressible flow

Data reduction of average friction factor of gas flow through adiabatic micro-channels

This paper presents data reduction of average friction factor of gas flow through adiabatic microchannels. In the case of micro-channel gas flow at high speed, the large expansion occurs near the outlet and the pressure gradient along the length is not constant with a significant increase near the outlet. This results in flow acceleration and a decrease in gas temperature. Therefore the friction factor of microchannel gas flow should be obtained with measuring both the pressure and temperature. The data reductions on friction factors were carried out under the assumption of isothermal flow for numerous experimental and numerical studies since temperature measurement of micro-channel gas flow at high speed is quite difficult due to the measurement limitations. In the previous study, it was found that the gas temperature can be determined by the pressure under the assumption of one dimensional flow in an adiabatic channel (Fanno flow). Therefore in the present study data reduction to estimate friction factors between two relatively distant points considering the effect of a decrease in temperature is introduced with the temperature determined by the measured pressure at a specific location. The Friction factors obtained by using the present data reduction are examined with the available literature and the results are compared with empirical correlations on Moody chart

Data reduction of average friction factor of gaseous flow in micro-channels with adiabatic wall

This study focuses on data reduction of average friction factor of gaseous flow through microchannels. In the case of microchannel gas flow at high speed, the large expansion occurs near the outlet and the pressure gradient along the length is not constant and increases near the outlet. This results in flow acceleration and a decease in bulk temperature. Therefore both pressure and temperature are required to obtain the friction factor of the microchannel gas flow. In the past data reduction of many experiments, the friction factors have been obtained under the assumption of isothermal flow since temperature measurement of compressible flow in micro-channels is quite difficult due to the experimental technique limitations. Kawashima and Asako [1] found that the gas temperature can be determined by the pressure under the assumption of one dimensional flow in an adiabatic channel (Fanno flow) to obtain the friction factor considering the effect of a decrease in gas temperature

Multivariable power least squares method. Complementary tool for response surface methodology

In Response Surface Methodology (RSM), variables are correlated through polynomial functions based on Stone-Weierstrass theorem. However, such formulation inherits four weaknesses: possible misleading approximation, incapability to accurately determine the ranking of factors' dominance, failure to analyse factors in random value and proliferation of guess functions due to Pascal Triangle. Therefore, this article aims to develop an improvised method to rectify and complement the weaknesses of RSM. Multivariable Power Least Squares Method (MPLSM) has been developed to correlate various sets of independent variables with dependent variable in the form of power functions. MPLSM is built upon least squares method, and able to approximate the indices of the variables easily. Two variants of MPLSM are suggested to further ensure the numerical stability: the Normalised MPLSM and Iterative MPLSM. The proposed method is not only substantial in big data analysis and multivariable problems, but also providing an alternative approach in engineering optimisation

Flow characteristics of gaseous flow through a microtube discharged into the atmosphere

Flow characteristics for a wide range of Reynolds number up to turbulent gas flow regime, including flow choking were numerically investigated with a microtube discharged into the atmosphere. The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The LB1 turbulence model was used in the turbulent flow case. Axis-symmetric compressible momentum and energy equations of an ideal gas are solved to obtain the flow characteristics. In order to calculate the underexpanded (choked) flow at the microtube outlet, the computational domain is extended to the downstream region of the hemisphere from the microtube outlet. The back pressure was given to the outside of the downstream region. The computations were performed for adiabatic microtubes whose diameter ranges from 10 to 500 µm and whose aspect ratio is 100 or 200. The stagnation pressure range is chosen in such a way that the flow becomes a fully underexpanded flow at the microtube outlet. The results in the wide range of Reynolds number and Mach number were obtained including the choked flow. With increasing the stagnation pressure, the flow at the microtube outlet is underexpanded and choked. Although the velocity is limited, the mass flow rate (Reynolds number) increases. In order to further validate the present numerical model, an experiment was also performed for nitrogen gas through a glass microtube with 397 µm in diameter and 120 mm in length. Three pressure tap holes were drilled on the glass microtube wall. The local pressures were measured to determine local values of Mach numbers and friction factors. Local friction factors were numerically and experimentally obtained and were compared with empirical correlations in the literature on Moody's chart. The numerical results are also in excellent agreement with the experimental ones

Computational analysis on the effect of size cylinder for the irreversible process in a piston-cylinder system using iced-ale method

A numerical analysis for the irreversible process in an adiabatic piston-cylinder system has been conducted. Two-dimensional compressible momentum and energy equations were solved numerically to obtain the state quantities of the system using the laminar flow model. The numerical method is based on the combined Implicit Continuous-fluid Eulerian technique and the Arbitrary-Lagrangian-Eulerian method. The computations were performed for a single compression process with the piston velocity of-10 m/s for the effect of diameter cylinder, D which are 0.02 m, 0.04 m and 0.06 m. We found that, size of the diameter cylinder has an effect to the occurrence of the irreversible process. Increase the size of diameter of the cylinder will resulting to the increases of the average pressure on the piston surface, pps/prev in the cylinder. The average value of pps/prev during the compression process for the case of the D = 0.02 m, 0.04 m and 0.06 m are 1.00026, 1.00042, and 1.00057, respectively

Revisiting tin melting for phase change model verification

Model verification is necessary before numerical models can be applied to produce meaningful results. For solid-liquid phase change modelling involving convection, pure gallium and tin melting have been widely used as reference for verification. It was later found that contrasting observations have been reported on the flow structure of both metals in the liquid region during the phase change process. Some researchers have reported monocellular while others reported multicellular structures in past works. In this work, tin melting problem was revisited by extending the results to flow structure visualization with Line Integral Convolution (LIC) plots to confirm the flow structure for tin melting thus pure metals in general. Enthalpy-porosity formulation coupled with Finite-Volume Method (FVM) was used to solve the set of governing equations which represented the problem at Prandtl Number = 0.02, Stefan Number = 0.01 and Rayleigh Number = 2.5 x 105. The location of solid-liquid interface and LIC plots at different times were presented. At initial state, the solid-liquid interface was closely similar for all grid sizes but as time progresses, finer grids provided improved solutions as expected. Reasonable fine grid size must be selected for solid-liquid phase change models to ensure complete physics of the problems are captured and eventually yield acceptable numerical results. The LIC plots confirmed that the flow structure is multicellular. Future phase change models referring to pure metal melting problem for verification should obtain similar flow structure to be considered acceptable

Identification of gas flow regimes in adiabatic microtubes by means of wall temperature measurements

There exists the laminar flow, transitional flow, turbulent flow and choked flow regimes in a microtube gas flow. Development of a non-invasive identification method of the flow regimes within a microdevice is expected. This paper demonstrated how the internal gas flow regimes can be identified by measuring the distribution of the external wall temperature of the microchannel along the flow direction. A series of experiments were conducted by using nitrogen as working fluid through a stainless steel micro-tube with an inner diameter of 523 μm and a fused silica micro-tube having a diameter of 320 μm. The experiments were performed by fixing the back pressure at the exit of the microchannel at the atmospheric value and by varying the inlet pressure in order to modify the gas flow regime. In order to measure the external wall temperature along the microtube, two or three bare type-K thermocouples with a diameter of 50 μm were attached to the micro-tube external surface by using a high conductivity epoxy. In the case of the microtube having a diameter of 523 μm, local pressures were measured at three local pressure ports along the microtube. The pressure ports were placed on the opposite side of the tube wall where three thermocouples were attached. The microtube external wall was thermally insulated with foamed polystyrene to prevent heat gain or loss from the surrounding. The experimental results show that the wall temperature decreases in the laminar flow regime, increases in the transitional flow regime, decreases in the turbulent flow regime and it stays nearly constants in the choked flow regime. The behavior of the average Fanning friction factor and the local Mach number can be explained by identifying the flow regime. It is clarified that the microtube external wall temperature is a reliable indicator of the flow regime

SNPInterForest: A new method for detecting epistatic interactions

<p>Abstract</p> <p>Background</p> <p>Multiple genetic factors and their interactive effects are speculated to contribute to complex diseases. Detecting such genetic interactive effects, i.e., epistatic interactions, however, remains a significant challenge in large-scale association studies.</p> <p>Results</p> <p>We have developed a new method, named SNPInterForest, for identifying epistatic interactions by extending an ensemble learning technique called random forest. Random forest is a predictive method that has been proposed for use in discovering single-nucleotide polymorphisms (SNPs), which are most predictive of the disease status in association studies. However, it is less sensitive to SNPs with little marginal effect. Furthermore, it does not natively exhibit information on interaction patterns of susceptibility SNPs. We extended the random forest framework to overcome the above limitations by means of (i) modifying the construction of the random forest and (ii) implementing a procedure for extracting interaction patterns from the constructed random forest. The performance of the proposed method was evaluated by simulated data under a wide spectrum of disease models. SNPInterForest performed very well in successfully identifying pure epistatic interactions with high precision and was still more than capable of concurrently identifying multiple interactions under the existence of genetic heterogeneity. It was also performed on real GWAS data of rheumatoid arthritis from the Wellcome Trust Case Control Consortium (WTCCC), and novel potential interactions were reported.</p> <p>Conclusions</p> <p>SNPInterForest, offering an efficient means to detect epistatic interactions without statistical analyses, is promising for practical use as a way to reveal the epistatic interactions involved in common complex diseases.</p