Nusselt Values Research Articles

Numerical Study Characteristics of Heat Transfer on Staggered Cylinder Arrangement

This research was conducted to examine the characteristics of heat transfer and heat flux fluctuations of staggered cylinder arrangement. The parameters in this study is Reynolds 8.8x104 , inlet velocity of 1 m/s and inlet temperature of 600C. Numerical model used is a 2D model-URANS turbulence model is k-omega SST. The results of this study indicate that increased flow velocity in the narrow area on the second row and third row caused by the staggered arrangement. This led to the distribution of the local Nusselt third highest row two and row after row of the lowest. Highest temperature distribution found in the area and the intensity of the turbulent wake in the third row higher value this is due to the influence of turbulence on the previous line and the arrangement of the cylinders. Effect of heat flux variation of the average Nusselt number appears lower in the heat flux 600 W/m2 than 200 W/m2 and 1000 W/m2 , but the third row average Nusselt values coincide with each other. While the average turbulent intensity increases towards the downstream. In the third row with a heat flux of 600 W/m2 has the highest turbulence intensity, while for the heat flux and heat flux of 200 W/m2 1000 W/m2 intensity turbulennya coincide with each other.

Effect of a circular cylinder on separated forced convection at a backward-facing step

The current study investigates the augmentation in the laminar forced convection characteristics of the backward-facing step flow in a two-dimensional channel by means of introducing an adiabatic circular cylinder in the domain. The effects of various cross-stream positions (i.e., y c = 0–1.5) of the circular cylinder on the flow and heat transfer characteristics of the backward-facing step flow has been numerically explored for the Reynolds number range 1–200 and Prandtl number of 0.71 (air). The governing continuity, Navier–Stokes and energy equations along with appropriate boundary conditions are solved by using FLUENT. The flow and thermal fields have been explained by streamline and isotherm profiles, respectively; however, no temperature dependency effects are considered for the flow viscosity and thermal conductivity. The engineering parameters like wake/recirculation length, total drag coefficient and average Nusselt number, etc. are calculated for the above range of conditions. The present results show an enhancement in the peak Nusselt value of up to 155% using a circular cylinder as compared to the unobstructed case (i.e., without cylinder). Finally, simple correlations for total drag coefficient and peak Nusselt number are obtained for the above range of conditions.

Thermal development of the laminar flow of a Bingham fluid between two plane plates with viscous dissipation

The thermal development of the hydrodynamically developing laminar flow of a viscoplastic fluid (fluid of Bingham) between two plane plates maintained at a constant temperature has been studied numerically. This analysis has shown the effect caused by inertia and the rheological behaviour of the fluid on the velocity, pressure and temperature fields. The effects of Bingham and Peclet numbers on the Nusselt values with the inclusion of viscous dissipation are also discussed.

Experimental Investigation of Turbulent Heat Transfer in the Entrance Region of Microchannels

Within the entrance region of a closed channel, the effects on flowfield and heat transfer mechanisms are significant. Where microchannels and microdevices are concerned, there are limited heat transfer studies into this region, and those available are for laminar flows. In the turbulent regime, where the hydraulic resistance is conventionally increased, higher pumping powers are required. However, future microscale applications may have a need for turbulent flow data in microchannels. There is currently no heat transfer data available on the turbulent entrance region of microchannels. An experimental investigation has been carried out to explore turbulent convection heat transfer in the entrance region of uniformly heated microtubes. The measurement of local wall temperatures is achieved through the use of unencapsulated thermochromic liquid crystals, a state-of-the-art, nonintrusive thermal-measurement technique. Heat transfer data was obtained for two stainless steel microtubes, with nominal inner diameters of 1.067 and 0.508 mm, over a Reynolds number range of 4000 to 9000. The working fluid is FC-72, and adequate tube entry length is provided for hydrodynamic flow development before heating. Local temperature data and Nusselt values are obtained in the thermal entrance region of the microchannels. The thermal turbulent entrance length is found to remain relatively constant for the Reynolds number range considered for both microtube diameters. This is in good agreement with conventional thermal turbulent entrance studies for pipes.

Two-Dimensional Analytical Model of Heat Transfer for Flames in Channels

A two-dimensional model for heat transfer in reacting channel flow with a constant wall temperature is developed along with an analytical solution that relates the temperature field in the channel to the flow Peclet number. The solution is derived from first principles by modeling the flame as a volumetric heat source and by applying jump conditions across the flame for plug and Hagen-Poiseuille velocity profiles and is validated via comparison with more detailed computational fluid dynamics solutions. The analytical solution provides a computationally efficient tool for exploring the effects of varying channel height and gas velocity on the temperature distribution in a channel in which a flame is stabilized. The results show that the Peclet number is the principal parameter controlling the temperature distribution in the channel. It is also found that although the Nusselt number is independent of the Peclet number (or velocity) in the postflame region, it can change by nearly 3 ord ers o f magnitude in the preflame region over the range of Peclet numbers (or velocities) expected in microcombustors. This has important implications for quasi-onedimensional numerical modeling of micro/mesoscale combustion, in which it is usual to select a single Nusselt value from the heat transfer literature.Acorrelation to facilitate incorporation of the streamwise Nusselt number variation is provided.

An Analysis of Film Condensation on a Horizontal Wire-Wrapped Tube

The paper gives a provisional analysis of heat transfer during laminar film condensation on a horizontal, wire-wrapped tube. This preliminary report is based on an approximate, partly empirical, earlier solution1. The present work includes capillary condensate retention between the wire and tube determined using the approach of Masuda and Rose2 for condensation on integral-finned tubes. The final result is given in the form of a simple equation for the enhancement ratio (heat transfer for the wire-wrapped tube divided by the Nusselt value for the plain tube with the same vapour-to-surface temperature difference). The theoretical enhancement ratio depends only on the tube and wire diameters and the pitch of the winding, as well as the surface tension and density of the condensate, and is independent of the vapour-to-surface temperature difference. The expression for enhancement may readily be used to optimize the geometric variables for condensation of a given fluid. Comparison with limited available experimental data is inconclusive and indicates the need for further experimental work and scrutiny of approximations used in the model.

Non-Newtonian fluid laminar flow and forced convection heat transfer in rectangular ducts

Numerical solutions for fully developed laminar flow forced convection heat transfer of a power law non-Newtonian fluid in rectangular ducts are presented. Finite difference methods are developed for the governing equations to obtain the velocity and temperature distributions. Friction factor results are given for flow through rectangular ducts of aspect ratios of 0.2, 0.5 and 1.0 with power law index n values of 0.5 to 1.0. For the same flow conditions the Nusselt values, maximum wall temperatures, and minimum wall temperatures for the H2 thermal boundary condition for different combinations of heated and adiabatic walls are obtained. Also the Nusselt values for slug flow ( n=0 ) are presented for the H2 boundary condition.

Influence of rheology on laminar heat transfer to viscoelastic fluids in a rectangular channel

Experimental studies of the laminar pressure drop and heat-transfer behavior of two types of aqueous polymer solutions were carried out in a 2:1 rectangular channel. The fluids studied were 1000 wppm of neutralized Carbopol 934 in deionized water and 1000 wppm of Separan AP-273 in tap water. Three difference thermal boundary conditions were studied. The experimental friction factors for the two polymer solutions agree with the value predicted for a purely viscous power law fluid. The measured Nusselt values for the two polymer solutions were considerably higher than the corresponding values for a power law fluid and higher than the experimental values for water. In this paper it is postulated that these high heat-transfer values are the result of secondary flows which arise from normal stress differences imposed on the boundaries of viscoelastic fluids in laminar flow through noncircular geometries. In addition, it is hypothesized that under laminar flow conditions the low frequency oscillatory behavior determines the relative elasticity, which in turn influences the heat-transfer performance of such fluids.

Laminar mixed convection heat transfer to viscoelastic fluids in a 5:1 rectangular channel

The fully established friction factors and heat transfer coefficients of the laminar flow of viscoelastic fluids in a 5:1 rectangular duct were measured experimentally. The bottom wall, one of the longer sides of the duct was heated by passing direct electric current through it, yielding constant peripheral wall temperature and constant heat input per unit length along the bottom wall. The other surfaces were adiabatic. The test section is 150 hydraulic diameters long. Aqueous solutions of two different polymers (polyacrylamide and hydroxyethylcellulose) were used as viscoelastic fluids. The measured laminar friction factors agree well with the Newtonian correlation when plotted as a function of Kozicki generalized Reynolds number (i.e., f=16/ Re ∗ ). The established laminar Nusselt values for aqueous solutions of hydroxyethylcellulose, a weakly elastic fluid, are not significantly different from the values for water when compared at about the same values of Prandtl number and Rayleigh number. The Nusselt numbers for polyacrylamide solutions are about 25% higher than those for water at comparable values of Prandtl number and Rayleigh number.

TURBULENT HEAT TRANSFER PERFORMANCE OF NEWTONIAN FLUIDS IN ASYMMETRICALLY HEATED RECTANGULAR CHANNELS

Abstract The fully established friction factors and heat transfer coefficients of the turbulent flow of water in a 150-hydraulic-diameters-long, 5:1 rectangular duct were measured experimentally. The bottom wider wall was heated by passing direct electric current through it, yielding constant wall temperature (spanwise) and uniform heat input per unit length along the bottom wall. The other surfaces were adiabatic. The measured turbulent Fanning friction factors are in good agreement with the Blasius equation. Spanwise-averaged, fully established turbulent local Nusselt values for water over a range of the Reynolds number from 10,000 to 50,000 for the case of asymmetric heating are about 11% below the values for symmetric heating. This observed decrease in the heat transfer is in good agreement with the available turbulent heat transfer results in asymmetrically healed rectangular channels. Assuming that the Dittus-Boelter equation yields a reasonable estimate of the turbulent Nusselt values in symmetrica...

TURBULENT HEAT TRANSFER AND PRESSURE DROP MEASUREMENTS FOR VISCOELASTIC FLOWS THROUGH RECTANGULAR PASSAGES

Abstract The fully established friction factors and heat transfer coefficients of the turbulent flow of viscoelastic fluids in a 5:1 rectangular duct were measured experimentally. The bottom wall, one of the longer sides of the duct, was heated by passing direct electric current through it, yielding constant peripheral wall temperature and constant heat input per unit length along the bottom wall. The other surfaces were adiabatic. The test section is 150 hydraulic diameters long. The fluids studied were aqueous solutions of three different polymers (potyacrylamide, polyethyleneoxide, and hydroxyethylcellulose). The measured turbulent asymptotic friction factors are in agreement with the values reported in the literature. However, the fully established turbulent Nusselt values do not appear to have reached the asymptotic limiting value for heat transfer. The available empirical j-factor correlation for circular pipes does not predict the present experimental values measured in a rectangular channel with only one wall being heated.

Free convection heat transfer from horizontal wires to pseudoplastic fluids

Experimental Nusselt values are reported for free convection heat transfer from small diameter horizontal wires to pseudoplastic fluids.

Heat transfer to water flowing turbulently through a rectangular duct with asymmetric heating

Experimental, fully established friction factors and Nusselt numbers are reported for the turbulent flow of water in a 2:1 rectangular duct. The measured friction factors are in good agreement with the Blasius prediction if the Reynolds number proposed by Jones is used. The measured Nusselt values are in good agreement with the results of Novotny et al. for the turbulent flow of air if the results are corrected for the effects of the Prandtl number. The influence of asymmetric heating on the average Nusselt number was found to be much less for water as a working fluid than for air. This difference is explained by the fact that water has a higher Prandtl number than air.

Slug flow heat transfer in ducts of regular polygonal cross section

Nusselt numbers for slug-flow heat transfer in regular polygonal ducts for the case of uniform wall temperature have been calculated by means of the point-matching method. The Nusselt values vary from 4.38 for the triangular duct up to 5.78 for the circular duct.

Heat Transfer From a Nonisothermal Disk Rotating in Still Air

The influence of a variation in the surface temperature on the heat transfer from a disk rotating in still air was studied. The temperature difference between the disk sur face and the fluid at rest was allowed to vary as a power function of the radius. The results are given as the ratio of the local Nusselt value for a power-function walltemperature variation to the isothermal Nusselt value. (C.J.G.)

Nusselt values for estimating turbulent liquid metal heat transfer in noncircular ducts

AbstractA simple expression for estimating the turbulent forced‐convection heat transfer performance of liquid metals flowing through noncircular ducts is presented. This equation requires the knowledge of the slug Nusselt number evaluated for the specific geometry and for the pertinent boundary conditions. Such Nusselt values are presented herein for a number of technically important geometries. One check on the heat transfer prediction given by Equation (4) is in the case of an annular duct with constant heat flow through the outer wall with the inner wall insulated, for which experimental data exist. The prediction agrees within 20% with the experimental data.Several possible boundary conditions that may exist in noncircular cross sections are throughly discussed, and it is hoped that as a result this paper may serve to clarify some of the confusion existing in the literature.

Heat Transfer to Lead-Bismuth and Mercury in Laminar and Transition Pipe Flow

Abstract Experimental heat-transfer results in the low Reynolds modulus range of 1000 to 10,000 are reported for two different liquid metals, lead-bismuth eutectic and mercury. The single heat exchanger used in both series of tests was a ¾-in., 18-gage, 4-ft-long mild-steel tube. Similar heat-transfer tests with water in laminar and turbulent flow were performed with this same tube before and after each liquid-metal series, and the agreement of the turbulent-water tests with previous published results supports the liquid-metals test procedure. The data for both liquid metals, including flow in the upward and downward directions, are correlated on a Nusselt-Peclet basis with a single curve representing all experimental results with a maximum deviation of 20 per cent. The experimental Nusselt values decrease from approximately 6 at Reynolds number of 10,000 down to 1 at a Reynolds number of 1200, and consequently are considerably lower than the theoretical constant heat-rate laminar-flow Nusselt value of 4.36 (6). Possible causes for this behavior which have been investigated and found to be inadequate when considered independently include: (a) Insulating gas-layer or oxide-film fouling at the heated surface; (b) nonwetting; (c) uncertainties in property values; (d) axial-conduction effects. Although no supporting data are available, a distortion of the velocity profile from the parabolic distribution appears to be the most plausible explanation.