Physical study of the non-equilibrium development of a turbulent thermal boundary layer

The direct numerical simulation of a non-equilibrium turbulent heat transfer case is performed in a channel flow, where non-equilibrium is induced by a step change in surface temperature. The domain is thus made of two parts in the streamwise direction. Upstream, the flow is turbulent, homogeneous in temperature and the channel walls are adiabatic. The inflow conditions are extracted from a recycling plane located further downstream, so that a fully developed turbulent adiabatic flow reaches the second part. In the domain located downstream, isothermal boundary conditions are prescribed at the walls. The boundary layer, initially at equilibrium, is perturbed by the abrupt change of boundary conditions, and a non-equilibrium transient phase is observed until, further downstream, the flow reaches a new equilibrium state, presenting a fully developed thermal boundary layer. The work aims at identifying the non-equilibrium effects that are expected to be encountered in comparable flows, while providing the means to understand them. In particular, the study allows for the identification of an inner region of the developing boundary layer where several quantities are at equilibrium. Other quantities, instead, exhibit a behaviour of their own, especially in proximity to the leading edge. The analysis is supported by mean and root-mean-square profiles of temperature and velocity, as well as by budgets of first- and second-order moment balance equations for the enthalpy and momentum turbulent fields.

CFD Analysis of the Optimization of Length of Capillary Tube for a Vapor Compression Refrigeration System

The capillary tube is commonly employed in refrigerant flow control systems. As a result, the capillary tube's performance is optimal for good refrigerant flow. Many scholars concluded performance utilising experimental, theoretical, and analysis-based methods. This paper examines the flow analysis of a refrigerant within a capillary tube under adiabatic flow circumstances. For a given mass flow rate, the suggested model can predict flow characteristics in adiabatic capillary tubes. In the current work, R-134a refrigerant has been replaced by R600a refrigerant as a working fluid inside the capillary tube, and the capillary tube design has been modified by altering length and diameter, which were obtained from reputable literature. The analysis is carried out using the ANSYS CFX 16.2 software. The results show thatutilising a small diameter and a long length (R600a refrigerant flow) is superior to the present helical capillary tube. The most appropriate helical coiled design with a diameter of 0.8 mm and a length of 3 m is proposed. Keywords: Capillary Tube, Condenser, Refrigeration effect, CFD.

Isentropic Compressor Efficiency Calculation Method for Small Gasoline Engine Turbocharger Using Supplier Performance Maps

A turbocharger efficiency performance map given by the supplier is calculated using adiabatic flow equations and non-adiabatic experimental data. The experimental data used for this calculation is measured in hot gas stand conditions which are not adiabatic and the efficiency calculation needs correction. This paper presents a method to correct the isentropic efficiency of a compressor using the supplier maps and a heat transfer model applied on the compressor. Water is circulating in the central housing to cool the turbocharger and this water flow could be considered as insulation for heat transfer between the compressor and the turbine. The thermal effect of the turbine on the compressor is then neglected and the compressor heat flux is calculated and used to correct the isentropic efficiency calculation. The heat transfer is considered between the compressor and the surrounding environment and between the compressor and the central housing. Experimental adiabatic measurements are used to validate the model. Experimental tests are carried with different oil and water temperatures combinations to test the accuracy of the heat transfer model with these different combinations.

Model of stationary gas network

Modern problems of quantitative risk assessment require a development of more sophisticated types of so-called formation models. The formation models give all the information about an accidental leakage due the depressurization needed for quantitative estimation of the dangerous substances accumulating in the environment and further calculation of impact factors on humans, buildings etc. Common type of depressurization is a release of gaseous substances throughout the accidental hole on the surface of apparatus or pipeline. The pipeline connecting two vessels and a hole occurred on the pipeline as well as streams of vapour phase moving inside the pipelines and throughout the hole is a classical example of graph theory transport network problem. Thereby the model of stationary gas network based on equations of subsonic and choked adiabatic flow (for ideal gas) with accounting of mixing processes has been proposed. The solution with applying of graph theory, linear algebra and numerical analysis has been found.The gas net is represented as an oriented graph with nodes as a pressure-points and lines as pipelines. Case of incorrect estimation of flows directions has been studied and the problem of solving algorithm’s self-correction has been arisen. The method of incidence matrix correction during solving process has been developed and applied. The Newton’s method of non-linear equations system solving has been applied and specific method of Jacoby matrix correction has been developed.The behaviour of gas network model has been studied on example of a hydrocarbons’ mixture leakage from an accidental hole on the pipeline connecting two vessels. Results of numerical simulation experiments showed good agreement of model with basic laws of ideal gas adiabatic flow movement and gas network system in general. The directions of flows were in agreement with pressures’ differences on the lines as well as material and energy conservation laws have been observed. Model can be applied in numerical risks analysis for modelling of accidental processes of gaseous substances leaks as well as for the transport problems of chemical technology or educational purposes.

On liquid phase hydrates formation in pipelines in the course of gas non-stationary flow

Nowadays, when the emphasis is on alternative means of energy, natural gas is still used as an efficient and convenient fuel both in the home (for heating buildings and water, cooking, drying and lighting) and in industry together with electricity. In industrial terms, gas is one of the main sources of electricity generation in both developed and developing countries. Pipelines are the most popular means of transporting natural gas domestically and internationally. The main reasons for the constipation of gas pipelines are the formation of hydrates, freezing of water plugs, pollution, etc. It is an urgent task to take timely measures against the formation of hydrates in the pipeline. To stop gas hydrate formation in gas transporting pipelines, from existing methods the mathematical modelling with hydrodynamic method is more acceptable. In this paper the problem of prediction of possible points of hydrates origin in the main pipelines taking into consideration gas non-stationary flow and heat exchange with medium is studied. For solving the problem the system of partial differential equations governing gas non-stationary flow in main gas pipeline is investigated. The problem solution for gas adiabatic flow is presented.

Numerical Simulations of Instability in the Shell of a Supernova Remnant Expanding in a Weakly in Inhomogeneous Interstellar Medium

Astronomical observations show that the supernova remnants, even with a close to spherical shape, usually have multiscale ripple-like distortions. For example 15 bends on the shock front are clearly visible in the remnant 0509-67.5. The global instability of the flow is considered as one of the possible mechanisms for generating such structures. In the frame of linear analysis [26] was shown that this instability has a resonance character. It means that the perturbations with a certain wavelength number should grow faster, therefore ripples in the remnant shell will manifest itself predominantly in a certain range of scales. In this paper we present the results of numerical simulations of the nonlinear stage of this instability, caused by small perturbations in the external environment, depending on their scale and intensity. The unpertubed gas is supposed to has a power-law spartial dependence ρ0(r) ~ r-ω, where ω is a constant. The blast wave generated by a supernova expolosion is descibed by a Sedov type similarity solution. We have developed two-dimensional numerical model of adiabatic flow with a blast wave in a comoving frame of reference based on parallel code AstroChemHydro [1]. It was shown that, according to the predictions of linear analysis, perturbations in the external flow amplify behind the front of the shock wave, which leads to the development of convective instability and the development of turbulence. The results of numerical simulations demonstrated that in shell-type flows (for omega = 2,7 and gamma = 4/3) external disturbances along with the characteristic rearrangement of the shock front and turbulization of the flow behind it, cause the formation of radially elongated filaments with a vortex structure behind the shock, the number of which is determined by the harmonic number of the perturbation l.

On Passing of Interstellar Clouds Through a Galactic Spiral Arm

It is believed that the taxonomy of interstellar clouds in their vicinity can serve as an indicator of the features of the geometry and intensity of galactic shock waves. In this paper, the authors present the results of a detailed two-dimensional hydrodynamic simulation of the passage of a cloud through the spiral arm of a galaxy and provide a brief analysis of the effects arising from this motion. The model of interstellar gas used assumes adiabatic flow in the spiral arm. The external gravitational field of the galactic disk and spiral arm is taken into account. The transverse dimensions of the arm in the calculations are taken as follows: the half-width of the arm is 1 kpc along the plane of the disk and 0.6 kpc in the vertical direction. A fragment of the flow is considered near and inside the spiral arm, the effects of the curvature of the arm and the influence of the Coriolis forces are neglected. It is shown that clouds passing through the arm are strongly deformed and lose a significant part of the mass or are completely destroyed in the case of low-mass clouds. The boundary value of the cloud mass at which complete destruction occurs lies in the interval between 3 000 and 6 000 M.