Heat Transfer Enhancement Effect Research Articles

Numerical study of periodically turbulent flow and heat transfer in a channel with transverse fin arrays

A numerical study has been conducted for the characteristics of the periodically fully developed turbulent flow and heat transfer in a channel with transverse opposite‐positioned fins. The Reynolds number range is 2 × 104 to 7 × 104. K‐ε model and wall function method were adopted during the calculation. The influence of the thermal boundary condition of the fin to the heat transfer has been verified. For the studied configuration the prominent feature that differs from the similar laminar heat transfer is the phenomenon of secondary peak of the Nusselt number distribution. Assessment of heat transfer enhancement under the constraint of the same pump power reveals that the effect of the configuration of the relative fin height, e/H, equal to 0.1 is superior to those of e/H equal to 0.15 and 0.2. Comparing with the results of the channel with rod disturbances, the studied configuration possesses nearly the same heat transfer enhancement effect. Transient simulations to cases with big fin have also been conducted to assure the validity of the steady algorithm.

Turbulent Heat Transfer Enhancement and Surface Heating Effect in Square Channels with Wavy, and Twisted Tape Inserts with Interrupted Ribs

Regionally averaged heat transfer distributions and friction factors in square channels with hemi-circular wavy tape, hemi-triangular wavy tape, twisted tape inserts, and twisted tape inserts plus interrupted ribs are investigated. The effect of surface heating condition on heat transfer enhancement is also investigated. Each wall of the square channel is composed of isolated copper sections. Regionally averaged Nusselt number ratio, channel averaged Nusselt number ratio, and friction factor ratio in turbulent air flows are presented for Reynolds numbers from 10,000 to 70,000. The results show that uneven surface heating enhances the heat transfer coefficient over uniform heating condition. Square channels with hemi-circular wavy tape (δ/D = 0.1) inserts produce a heat transfer coefficient three times higher than twisted tape inserts (H/D = 6) with almost 25 times the pressure drop penalty. Square channels with hemi-triangular or hemi-circular wavy tape (δ/D = 0.25) inserts produce almost a 2.5 times higher heat transfer coefficient than twisted tape inserts with 5 to 6 times pressure drop penalty. Square channels with twisted tape inserts plus interrupted ribs (e/D = 0.33) produce two times the heat transfer augmentation with four times pressure drop penalty over twisted tape inserts only. Square channels with twisted tape inserts plus interrupted ribs (e/D = 0.33) show the best overall heat transfer performance over other configurations.

Characteristics of Natural Convection Heat Transfer in a Oriented Horizontal Annuli with Heated Outer Tube and Cooled Inner Tube. (In the Case of the Elliptical Outer Tube).

This study was researched by a numerical analysis and experiment, on a natural convection heat transfer in a oriented horizontal elliptical annuli between heated outer tube and cooled inner tube. The outer tube was elliptical tube, and inner tube was cylindrical tube. It was experimented with the outer tube oriented to vertical side, slant upper side 45 degrees, horizontal side, respectively. The numerical analyses were performed by changing the elliptical ratio, inner tube radius, oriented angle, and Rayleigh number as parameters. Results obtained for different conditions are presented as temperature profiles, flow patterns, local Nusselt number, and total heat transfer. It was presented that the heat transfer enhancement effect or heat transfer obstruction effect for the total heat transfer by oriented angle.

Numerical Analysis of Natural Convection of Thermoelectrically Conducting Fluids in a Cubic Cavity under a Constant Magnetic Field. 3rd Report. Heat Transfer Enhancement Effect, Discrete Del Operator Method.

Generally speaking, the finite-element method in computational fluid dynamics is universally accepted, however computation by the CPU is time-consuming and requires large memory capacity for data storage. Therefore development of an analytical formulation to reduce the time and storage required for calculation is desired. In this paper, we propose a novel discrete del operator method in order to overcome these defects. This method is formulated using the discrete del operator as the element coefficient matrices in finite-element analysis, and low-memory and high-speed calculations are carried out. In particular, we examine whether this method is effective for the numerical analysis of natural convection of thermoelectrically conducting fluids in a cubic cavity. Moreover, we estimate the effect of heat transfer enhancement under a weak magnetic field on the Hartmann number.

Heat transfer to non-Newtonian fluids in laminar flow through rectangular ducts

Heat transfer to non-newtonian fluids flowing laminarly through rectangular ducts is examined. The conservation equations of mass, momentum, and energy are solved numerically with the aid of a finite volume technique. The viscoelastic behavior of the fluid is represented by the Criminale-Ericksen-Filbey (CEF) constitutive equation. Secondary flows occur due to the elastic behavior of the fluid, and, consequently, heat transfer is strongly enhanced. It is observed that shear thinning yields negligible heat transfer enhancement effect, when compared with the secondary flow effect. Maximum heat transfer is shown to occur for some combinations of parameters. Thus, there are optimal combinations of aspect ratio and Reynolds numbers, which depend on the fluid's mechanical behavior. This result can be usefully explored in thermal designs of certain industrial processes.

The effect of a magnetic field on heat transfer in a slotted channel

The results of numerical and experimental studies of liquid metal heat transfer in slotted channels in a transverse magnetic field are presented. Test results showed an improvement in heat transfer in a straight channel at low and moderate interaction parameter N . The Nusselt number at small N (around 120) was up to twofold higher than in turbulent flow without a magnetic field, the Peclet number being equal. This effect of heat transfer enhancement is caused by the generation and development of large-scale velocity fluctuations close to the heated wall area. Qualitative and quantitative correlations between heat transfer and velocity fluctuation characteristics are presented.

Surface evaporation of liquid flowing down an inclined groove

The heat transfer enhancing effect of a groove has long been applied to such operations as evaporation and condensation, but qualitative estimation of the mechanism has not been sufficiently analyzed. This paper presents a study on surface evaporation of liquid flowing down an inclined groove cutting upon a 34 mm thick and 500 mm long cupro-nickel plate heated by an electric plate heater. Temperature distributions of liquid and the metal plate were precisely measured. Simultaneously, a simulation program with the finite element method was developed to calculate the convective flow and temperature distributions with respect to a cross section and the longitudinal change. Then, the heat transfer enhancement effect was qualitatively elucidated for various flow conditions by comparing the calculated result with measured temperature distributions. An application of the result to the roof style desalting solar distiller is discussed.

Convection From a Periodically Stretching Plane Wall

This paper documents the convection heat transfer enhancement effect that is associated with the periodic (timewise and spatial) longitudinal stretching of an infinite plane wall bathed by a semi-infinite fluid reservoir. It is shown that the flow field reaches a truly periodic state, and that the temperature field does not. The timedependent heat transfer rate is greater than the classical pure-diffusion limit only when the Reynolds number based on wall speed and wavelength is greater than 1. The enhancement effect becomes pronounced as Re increases above 10. The enhancement effect increases also as the Prandtl number increases, provided Re is greater than 1. The same conclusions apply in the case of convective mass transfer, when a concentration difference is maintained between the flexible wall and the ambient fluid.

Investigation of local heat transfer associated with the evaporation of a liquid film on a horizontal ribbed tube

The heat-transfer enhancement effect of longitudinal ribs on a horizontal tube is discussed on the basis of experimental data on the local heat-transfer Co-efficients.

The effect of surface renewal due to largescale eddies on jet impingement heat transfer

The mechanism for the enhancement of stagnation-point heat transfer was explored analyzing the large-scale turbulent structures of an impinging round jet by a statistical technique with conditional sampling. It has been found that large-scale eddies impinging on the heat transfer surfaces produce a turbulent surface-renewal effect dominant for the enhancement of the jet impingement heat transfer. The effect of heat transfer enhancement can be described in terms of the turbulent Reynolds and Strouhal numbers based on the characteristic turbulence intensity and frequency of the large-scale eddies impinging on the stagnation-point boundary layer.

Enhancement of radiation heat transfer based on the non-gray feature of radiative gases. (3rd report CO2 gas in the laminar-flow system)

The enhancement effect of radiation heat transfer based on the multibands feature of radiative gas is analysed theoretically for the system of laminar-channel flow of high temperature C02 gas between two parallel heat absorbing surfaces. An integro-differential equation of energy conservation, which takes into account the multi-bands feature of the radiative gas is solved numerically, and the enhancement effect of heat transfer by installing a solid plate in radiative gas is calculated. From these calculations, it is shown that the enhancement of local heat flux by as much as 80% is obtained and that this enhancement effect is caused by the very high radiative heat flux from the solid surface to the heat absorbing surface, notwithstanding the slight decrease in the radiative heat flux from the gas by installing a solid plate. It is also made clear that the analysis using the conventional gray gas assumption overestimates the decrease in the radiative heat flux from the gas, and thus estimates a too small enhancement effect.

Role of large-scale coherent structures in impinging jet heat transfer.

A large-scale coherent structure responsible for the enhancement of stagnation-point heat transfer was successfully determined using a statistical analysis with conditional sampling in an impinging round jet. It was found that the optimal nozzle-to-plate spacing for maximal stagnation-point heat transfer is identical with the jet development length for a strong turbulent stream carrying large-scale energy-containing eddies. A physical model of heat transfer enhancement was experimentally derived from the viewpoint of turbulent surface-renewal induced by large-scale eddies impinging on the stagnation-point boundary layer. It was found that the effect of heat transfer enhancement is a function of the proposed surface-renewal parameter consisting of the characteristic turbulence intensity and frequency of the large-scale eddies.

A vacuum technic for the chemist

A Transport Membrane Condenser (TMC), made from nanoporous membrane tube bundles, was developed to recover the water vapor and its significant amount of latent heat from boiler flue gases to improve boiler efficiency and save water. Experiments have been carried out to study the phenomena for both a nanoporous membrane tube bundle and an impermeable stainless steel tube bundle with the same characteristic dimensions. Flue gas streams with water vapor mass fraction 11.3%, temperature range from 65 °C to 95 °C are used for the experimental study, which covers the typical TMC waste heat recovery application parameter ranges. Results show convection Nusselt numbers of the membrane tube bundle are 50–80% higher than that of the impermeable stainless steel tube bundle at typical condensation heat transfer conditions. The parametric study has been done by varying cooling water inlet flow rate, water inlet temperature, flue gas inlet flow rate, inlet temperature, and inlet dew point. The condensing heat transfer enhancement effect gives a good perspective for using nanoporous membrane surface to design high efficiency condensing heat exchangers to recover both water vapor and its latent heat from high moisture content low grade waste heat streams.