Cheng-Xian Lin

Developing turbulent convective heat transfer in helical pipes

Abstract A fully elliptic numerical study has been carried out to investigate three-dimensional turbulent developing convective heat transfer in helical pipes with finite pitches. The κ-e standard two-equation turbulence model is used to simulate turbulent flow. The governing equations are solved by a control-volume finite element method. Numerical results have been compared to the existing experimental data and a good agreement has been obtained. The results presented in this paper cover a Reynolds number range of 2.5 x 104∼ 1.0 x 105, a pitch range of 0.0 ∼ 0.6 and a curvature ratio range of 0.025 ∼ 0.050. The developments of effective thermal conductivity and temperature fields and local and average Nusselt numbers are given and discussed. It has been found that the examined parameters exert complex effects on developing thermal fields and heat transfer in the helical pipes. The Nusselt numbers for the helical pipes are oscillatory before the flow is fully developed, especially for the case of relatively large curvature ratio.

Numerical investigations of liquid-solid slurry flows in a fully developed turbulent flow region

In this paper, a simplified 3D algebraic slip mixture (ASM) model is introduced to obtain the numerical solution in sand–water slurry flow. In order for the study to obtain the precise numerical solution in fully developed turbulent flow, the RNG K–e turbulent model was used with the ASM model. An unstructured (block-structured) non-uniform grid was chosen to discretize the entire computational domain, and a control volume finite difference method was used to solve the governing equations. The mean pressure gradients from the numerical solutions were compared with the authors experimental data and that in the open literature. The solutions were found to be in good agreement when the slurry mean velocity is higher than the corresponding critical deposition velocity. Moreover, the numerical investigations have displayed some important slurry flow characteristics, such as volume fraction distributions, slurry density, slip velocity magnitude, slurry mean velocity distributions, and slurry mean skin friction coefficient distributions in a fully developed section, that have never been displayed in the experiments. 2003 Elsevier Science Inc. All rights reserved.

Condensation of R134a flowing inside helicoidal pipe

Condensing heat transfer and pressure drop characteristics of an ozone-friendly refrigerant HFC-134a (hydrofluorocarbon 134a) flowing inside a 12.7 mm helicoidal tube were investigated experimentally to obtain heat transfer data and correlations. For long helicoidal pipe, heat transfer measurements were performed for the refrigerant flow mass fluxes from 100 to 400 kg/m 2 /s, in the cooling water flow Reynolds number range of 1500< Rew < 9000 at fixed system temperature (338C) and cooling tube wall temperature (128C and 228C). With the increase of mass flux, the overall condensing heat transfer coeAcients of R134a increased, and slowly the pressure drops also increased. However, with the increase of mass flux (or the cooling water flow Reynolds number), the refrigerant side heat transfer coeAcients decreased. The eAects of cooling wall temperature on heat transfer coeAcients and system pressure drops were considered. Predictive correlations valid over the above water flow Reynolds number ranges and refrigerant flow mass flux were proposed. Helicoidal pipe heat transfer characteristics were compared with data for horizontal straight pipe from literature reports. 7 2000 Elsevier Science Ltd. All rights reserved.

Combined laminar forced convection and thermal radiation in a helical pipe

Abstract The interaction phenomena between laminar forced convection and thermal radiation in a participating medium inside a helical pipe were studied numerically. The P-1 radiation model was applied to the combined convection–radiation simulations. The three-dimensional governing equations for laminar flow and heat transfer were solved with a control-volume finite difference method (CVFDM) with second-order accuracy, and the O-type structure grid was adopted in this study. The effects of thermal radiation on the convective flow and heat transfer were measured by comparing the numerical results with and without thermal radiation. The comparison showed that under the conditions examined in this paper, although the thermal radiation had no significant influence for flow and temperature fields, especially in a fully developed region, it substantially enhanced the total heat transfer in the helical pipe.

Laminar forced convection in the entrance region of helical pipes

Abstract In the present study, three-dimensional laminar forced flow and heat transfer in the entrance region of helical pipes have been investigated using a fully elliptic numerical method. Laminar flow and heat transfer were assumed to develop from the inlet to the outlet simultaneously. The governing equations were solved by means of a control-volume finite element method. The results presented in this paper cover a Reynolds number range of 250–2000, a pitch range of 0.0–0.6, and a curvature ratio range of 0.025–0.20. The present elliptic numerical results are compared with previous experimental data and parabolic numerical data. The developments of temperature field, main and secondary velocity fields, local and average friction factors, and local and average Nusselt numbers are given and discussed. It has been found that both the friction factor and Nusselt number are oscillatory in the entrance region of helical pipes. The pitch and Reynolds number exert different effects on the developments of the friction factor and Nusselt number than the curvature ratio.

Turbulent heat transfer to near-critical water in a heated curved pipe under the conditions of mixed convection

Abstract Numerical modeling was performed to investigate the developing turbulent flow and heat transfer characteristics of water near the critical point in a curved pipe. The renormalization group (RNG) κ – ϵ model was used to account for the turbulent flow and heat transfer in the curved pipe at a constant wall temperature with or without buoyancy force effect. A control volume finite element method (CVFEM) was used to solve the three-dimensional full elliptic governing equations for the problem numerically. Due to the great variation in physical properties of water near the critical point, the turbulent flow heat transfer can be significantly altered compared with the pure forced convection in the curved pipe. This study explored the influence of the near-critical pressure and wall temperature on the development of fluid flow and heat transfer along the pipe. The numerical results for forced convective flow and heat transfer were compared with experiments available in the literature. Based on the results of this research, the velocity, temperature, heat transfer coefficient, friction factor distribution, and effective viscosity are presented graphically and analyzed.

The effects of inlet turbulence on the development of fluid flow and heat transfer in a helically coiled pipe

Abstract In this paper, the effects of inlet turbulence level on the development of three-dimensional turbulent flow and heat transfer in the entrance region of a helically coiled pipe are investigated by means of a fully elliptic numerical study. The k – e standard two-equation turbulence model is used to simulate turbulent flow and heat transfer, which are assumed to develop from the inlet to the outlet simultaneously. Constant wall temperature and uniform inlet conditions are applied. The governing equations are solved by a Control-Volume Finite Element Method with an unstructured nonuniform grid system. Numerical results presented in this paper cover a Reynolds number range of 10 4 –10 5 , a Prandtl number range of 0.02–100, and an inlet turbulence intensity range of 2–40%. The development of the main velocity field, the secondary velocity field, the temperature field, bulk turbulent kinetic energy, the average friction factor, and average Nusselt numbers are discussed. It is found that bulk turbulent kinetic energy far from the entrance is not affected by the inlet turbulence level. Significant effects of the inlet turbulence level on the development of the friction factor and Nusselt number occur within a short axial distance from the entrance only.

COMBINED TURBULENT FORCED CONVECTION AND THERMAL RADIATION IN A CURVED PIPE WITH UNIFORM WALL TEMPERATURE

The combined turbulent forced-convective and radiative heat transfer of a participating medium in the entrance region of a curved pipe is investigated numerically. The P-1 radiation model and the renormalization group (RNG) k- k model are applied to simulate the turbulent convective heat transfer with thermal radiation in the curved pipe at a constant wall temperature. A control-volume finite-element method (CVFEM) with second-order accuracy is used to solve the governing equations. This study explores the interaction phenomena between turbulent forced convection and thermal radiation in a participating medium inside a curved pipe at different temperature ratio, optical thickness, and wall emissivity.

Turbulent mixed convective heat transfer in the entrance region of a curved pipe with uniform wall-temperature

Abstract A fully elliptic numerical study was performed to investigate the buoyancy-affected, three-dimensional turbulent flow and heat transfer in the entrance region of a curved pipe. The renormalization group (RNG) k–e model was used to simulate the turbulent flow and heat transfer in the pipe. A dimensionless ratio parameter, Gr \D n 1+Pr , was employed to characterize the relative magnitude of buoyancy and centrifugal effect on the secondary flow in the curved pipe. The computed results for forced convective flow and heat transfer show agreement with previous experimental data. It was found that the distribution of axial velocity and temperature rotated clockwise to a different extent depending on the ratio of Gr \D n 1+Pr . At higher Grashof numbers, the developing secondary flow field showed the existence of three vortices. The peripherally averaged Nusselt number and friction factor exhibit oscillatory behavior along the streamwise direction. The augmentation of the average Nusselt number and friction factor resulting from buoyancy was prominent at the entrance region of the pipe, but gradually became weaker further downstream.

Study of double-diffusive velocity during the solidification process using particle image velocimetry

Abstract This paper reports the velocity distribution of double-diffusive convection of a binary mixture in a rectangular enclosure during the solidification process. The mixture was a NH4Cl–H2O solution. The advanced technique, particle image velocimetry (PIV), was used to measure the velocity distribution in the liquid region during solidification. For the purpose of comparison, the solidification of pure water was studied with the same technique. The temperature of the cooling walls in the test chamber and the temperature of the test solution during solidification were also measured. The double-diffusive flow was found to be stronger at the beginning of solidification; the flow decays as solidification proceeds. The velocity distribution of the hypereutectic solution of NH4Cl–H2O has evident difference in comparison with hypoeutectic solution.