Effects Of Variable Physical Properties Research Articles

Theoretical and Experimental Study of Coiled Finned tube Heat exchanger for Helium Liquefaction Plant

The performance of a liquefier is governed by the effectiveness of its heat exchangers. Coiled finned tube heat exchangers are used in medium capacity helium liquefiers based on Collins cycle. Experimental and theoretical analysis of the first heat exchanger in helium liquefaction cycle is presented in this work. The objective of this study is to design, optimize and test the heat exchanger to achieve the desired effectiveness and therefore the drop in the temperature of the gas on the hot side. A transient numerical model is employed to optimize the geometrical configuration and operating parameters of the coiled finned tube heat exchanger. Effect of variable physical properties, axial conduction and radiative heat transfer of the outer shell is also considered. The numerical model is validated with experimental results found in open literature. The numerical model is then used for deriving the optimum geometrical configuration of the coiled heat exchanger. Experimental observations from the tests carried out on a fabricated prototype are also reported in this work. Finally, results of the numerical model are also compared with the experimental results obtained in this study.

Chebyshev collocation spectral method for radiative transfer in participating media with variable physical properties

Chebyshev collocation spectral method based on discrete ordinates equation is employed to solve radiative heat transfer problems in participating media with variable physical prosperities (including space-dependent or temperature-dependent refractive index, absorption coefficient and scattering coefficient). Discrete ordinates method is employed to discretize the solid angle domain. Chebyshev polynomial and collocation spectral method are adopted to express and discretize space domain, respectively. Numerical results obtained by the Chebyshev collocation spectral-discrete ordinates method (CCS-DOM) are presented in this paper and the results show the CCS-DOM has a good accuracy and efficiency for radiative heat transfer problems in participating media. At last, the effects of variable physical properties on radiative heat transfer are analyzed and it can be found that the distributions of refractive index, absorption coefficient and scattering coefficient have a significant effect on radiative transfer and energy distribution.

Natural convection heat transfer of molten salt in a single energy storage tank

Heat transfer in molten salt in a cylinder tank is studied via simulation and experiment to obtain its natural convection heat transfer in a single energy storage tank. Simulation and experimental results show that the natural convection heat transfer of water in a cylinder tank fits well with Garon’s correlation. However, significant deviations occur when Garon’s correlation is used to predict the natural convection heat transfer of molten salt because of its high viscosity and low thermal conductivity. However, the simulated data of the natural convection heat transfer of molten salt fit well with those of the experimental results. Thus, a correlation that considers the effect of variable physical properties is proposed in this study to predict the natural convection heat transfer of molten salt. The deviation of the present data from the proposed correlation is less than ±20%. The results of this study can serve as a basis for the design of single energy storage tanks.

Heat transfer characteristics of developing gaseous slip-flow in rectangular microchannels with variable physical properties

The effects of variable physical properties on the flow and heat transfer characteristics of simultaneously developing slip-flow in rectangular microchannels with constant wall temperature are numerically investigated. A colocated finite-volume method is used in order to solve the mass, momentum and energy equations in their most general form. Various channel aspect ratios are studied at different Knudsen numbers. Simulations indicate that the constant physical property assumption can result in under/over-prediction of the local friction and heat transfer coefficients depending on the problem configuration. Density and thermo-physical property variations have significant effects on predicting flow and heat transfer characteristics in the developing and fully-developed regions. The degree of discrepancy varies for different cases depending on Knudsen number, aspect ratio and the temperature difference between the channel inlet and the wall. The results suggest that even low temperature differences can alter the friction and heat transfer coefficients considerably.

Numerical study of BSA ultrafiltration in the limiting flux regime — Effect of variable physical properties

Ultrafiltration of BSA solutions in a parallel cell was studied by numerical methods taking into account concentration dependent properties. The Navier–Stokes equations, in the stream function–vorticity formulation, and the mass transport equation were solved simultaneously. The numerical code was validated by comparison with data from benchmark analytical solutions. The flow and concentration fields in the limiting flux regime were studied in detail and compared with the fields for constant properties. Numerical results were also compared with experimental data of the permeate velocity in the limiting flux regime published in the literature. This comparison shows that the concentration dependence of the viscosity, diffusivity and osmotic pressure are all necessary to explain experimental results published in the literature.

Numerical simulation on the falling film absorption process in a counter-flow absorber

Falling liquid film is commonly employed in variety of industrial systems, such as LiBr/H 2O absorption heat pump or chiller. In this paper, the falling film absorption in aqueous lithium bromide solution was investigated numerically using CFD software package-Fluent. The practical convective boundary condition at the cooling water side was considered. The heat transfer coefficient is assumed constant, and the coolant temperature changes linearly along its flow path. The numerical results indicate that the profile of temperature is exponential and their gradients are high due to the distinct heat effect associated with the absorption at the interface and the cooling effect of coolant at the wall at small downstream distance. As the downstream distance increases, the profile of temperature is nearly linear. The absorption heat and mass fluxes reach a maximum at the inlet region and decrease at the outside of the inlet region. Specially, the effect of variable physical properties on the absorption process was considered and discussed. The prediction of the total absorption mass transfer rate is about 6.5% higher when assuming constant properties.

Numerical prediction of turbulent convective heat transfer in mini/micro channels for carbon dioxide at supercritical pressure

A new approach to take into account the effects of variable physical properties on turbulence is suggested. It allows to choose freely the turbulent closure model for conventional terms due to velocity fluctuations and to describe coherently the additional terms due to density fluctuations. Numerical calculations based on the suggested approach have been performed for carbon dioxide flowing within mini/micro channels under cooling conditions. The numerical predictions show that the effects due to density fluctuations are smaller than it could have been initially supposed and that the heat transfer impairment for mini/micro channels, which some experiments seem to highlight, is not completely explained by the considered model.

Turbulent momentum and heat transfer in channel gas flow at high heat loads: A review

Recent experimental and computational works on the effects of variable physical properties of gas heat carriers on local heat transfer in channels of different cross sections, on turbulent flow structure, and on the heat transfer mechanism in the stabilized heat transfer region in a tube are reviewed. Recommendations are given with respect to the effect of the variable physical properties of a gas on local heat transfer in smooth or helical tubes and in annular channels equipped with smooth, rough, or helical inner tubes over wide ranges of geometrical and performance parameters. Relations are presented to describe velocity and temperature profiles in smooth tube flow of a gas with variable physical properties. Problems for future study are formulated.

Effects of variable physical properties on heat transfer in free convection around a horizontal cylinder

Theoretical and experimental studies have been made on the effects of variable viscosity and compressibility on the average heat-transfer coefficient.

Heat transfer and friction of rough ducts carrying gas flow with variable physical properties

The paper presents the results of an experimental investigation into the local heat transfer and friction of gas (air) flow in annuli with the inner rough tube for the following ranges of parameters: Re = 5 × 10 3−5 × 10 5, temperature factor T w / T f = 1−2.8, relative height k/ d e = 0.0028−0.021 and nearly optimum relative pitch s/ k ~ 10 of rectangular and rounded trapezoidal roughness elements. The correlations have been developed to calculate the heat transfer and friction of rough annuli with allowance for the effect of variable physical properties of a coolant and also the relationships for the hydrodynamic and thermal functions of roughness to determine the thermohydraulic characteristics of rough tube clusters. It has been established that, in contrast to smooth tubes, the effect of variable physical properties of the gas on heat transfer in rough tubes depends on k/ d e and Re and diminishes with the increase of these parameters.

Effects of Variable Physical Properties on Free Convection in a Porous Insulating Layer

A numerical study is made of the effects of variable properties on free convection in vertical, 'herium-(or air-) saturated porous layer. The temperature differences between hot and cold boundary walls are as large as 1000°C. The effects of variations in viscosity, thermal conductivity and density are included. Flow patterns are found to be considerably distorted in a horizontal direction to the symmetrical patterns predicted by the constant-property theory, whereas the overall heat-transfer characteristics do not alter very much provided that representative properties are evaluated at a mean temperature. At sufficiently large amplitude of convection, however, the deviation from the Boussinesq approximation is not negligible.

Effects of Variable Physical Properties on Free Convection in a Porous Insulating Layer

空気(あるいはヘリウム)で満たされた垂直断熱層内の自然対流に及ぼす物性値変化の影響を,高温面温度1000℃までの範囲で粘性係数,熱伝導率,密度の温度変化を考慮し,数値解析を行った.物性値変化を考えない場合に比べて対流パターンはかなり大きなひずみを生じるが,熱伝達に関しては平均温度における代表物性値で整理すると定物性のときと大差ないことが示される.

Effects of variable physical properties in laminar flow of pseudoplastic fluids

Effects of variable physical properties in laminar flow of pseudoplastic fluids

Effects of variable physical properties on laminar film condensation of saturated steam on a vertical flat plate

A theoretical study is made of steady two-dimensional laminar film condensation of pure saturated steam at 100°C and atmospheric pressure on an isothermal vertical wall. The variable physical properties of the liquid condensate are taken to be those of saturated water at the appropriate temperatures. On neglecting the effects of surface tension at the liquid-vapour interface the governing equations for the liquid and vapour phases are simplified, using boundary-layer approximations, and admit to a solution in terms of similarity variables. From numerical solutions of the resulting ordinary differential equations flow characteristics for the liquid and vapour phases are determined for the wall temperatures 0, 10, 40, 70 and 90°C. The related results on wall heat transfer and mass flow in the liquid phase are presented and compared in detail with those from existing theories.