Prediction Of Heat Transfer Rate Research Articles

Nusselt number correlations for heat transfer to small spheres in thermal plasma flows

Seven different equations predicting heat transfer rates to small spheres in plasma flows are examined considering argon and nitrogan as plasma gases from 300 to 21,000 K at 1 atm. For argon there is a general consensus up to 9000 K, beyond which wide deviations in behavior occur. For nitrogen, the seven correlations are in good agreement up to 4000 K, but show substantial deviations beyond this value. Comparisons with the sparsely available experimental data are made for argon from 300 to 17,000 K and for nitrogen up to 5500 K. Disagreement between the various correlations and experiment can exceed one order of magnitude.

Unsteady heat convection over circular cylinders

AbstractNumerical calculations of the velocity and temperature fields were conducted for unsteady flows over a single circular cylinder and a bundle of cylinders. Periodic disturbances, natural or externally imposed, were considered. Periodic and averaged heat transfer rates were examined. Results compare favorably with experimental data. Calculations were carried out to predict heat transfer rates in regimes of unsteady flows that have not been tested experimentally.

Calculation of Heat Transfer to Convection-Cooled Gas Turbine Blades

A mathematical model is presented for calculating the external heat transfer coefficients around gas turbine blades. The model is based on a finite-difference procedure for solving the boundary-layer equations which describe the flow and temperature field around the blades. The effects of turbulence are simulated by a low-Reynolds number version of the k-ε turbulence model. This allows calculation of laminar and transitional zones and also the onset of transition. Applications of the calculation method are presented to turbine-blade situations which have recently been investigated experimentally. Predicted and measured heat transfer coefficients are compared and good agreement with the data is observed. This is true especially for the pressure-surface boundary layer which is of a rather complex nature because it remains in a transitional state over the full blade length. The influence of various flow phenomena like laminar-turbulent transition and of the boundary conditions (pressure gradient, free-stream turbulence) on the predicted heat transfer rates is discussed.

A moving‐front transition boiling model

AbstractIn accordance with visual observations, a transition boiling model that assumes a quench front which alternately advances and retreats in the water flow direction, is proposed. With appropriate choice of a quench zone width and heat transfer rate when the surface is wet, the model predictions are in the same range as the observed temperature oscillations. An extension of the model leads to heat transfer rate predictions in general agreement with experimental observations.

Prediction of bubble growth rates and departure volumes in nucleate boiling at isolated sites

A model has been developed to predict heat transfer rates and sizes of bubbles generated during nucleate pool boiling. This model assumes conduction and a natural convective heat transfer mechanism through the liquid layer under the bubble and transient conduction from the bulk liquid. The temperature of the bulk liquid in the vicinity of the bubble is obtained by assuming a turbulent natural convection process from the hot plate to the liquid bulk. The shape of the bubble is obtained by equilibrium analysis. The bubble departure condition is predicted by a force balance equation. Good agreement has been found between the bubble radii predicted by the present theory and the ones obtained experimentally.

Liquid Crystal Visualization of Surface Heat Transfer on a Concavely Curved Turbulent Boundary Layer

An experimental heat transfer study on a concavely curved turbulent boundary layer has been performed. A new, instantaneous heat transfer measurement technique utilizing liquid crystals was used to provide a vivid picture of the local distribution of surface heat transfer coefficient. Large scale wall traces, composed of streak patterns on the surface, were observed to appear and disappear at random, but there was no evidence of a spanwise stationary heat transfer distribution, nor any evidence of large scale structures resembling Taylor-Gortler vortices. The use of a two-dimensional computation scheme to predict heat transfer rates in concave curvature regions seems justifiable.

Application of Rayleigh-Ritz method to forced convection turbulent heat transfer

An analytical model to predict heat transfer rates to an incompressible fluid in turbulent flow, with fully developed velocity profile, between a heated plate and a parallel, insulated plate is developed. The model employs van Driest's mixing length expression near the wall, a constant eddy diffusivitiy in the core and a constant turbulent Prandtl number. An approximate solution obtained by employing Rayleigh-Ritz method is shown to compare well with the ‘exact’ solution obtained by numerical integration of the differential equations. The results are compared with the available experimental data and analytical solutions.

Heat transfer from a buoyant smoke layer beneath a ceiling to a sprinkler spray. 2—an experiment

AbstractAn experiment is described which measures the heat transferred from a hot, flowing, buoyant gas layer in a mall, to a sprinkler spray. Interpolation techniques are described which allow for the large heat losses occurring within the apparatus (a large‐scale model mall). The results are compared with predictions based on the method developed in Part 1 and found to be consistently 150% of the predicted heat transfer rates. Given the assumptions required by the predictive calculations this is thought to be reasonable degree of agreement. The largest heat transfer rate to the one sprinkler spray in this experiment was 157 kW ± 38 kW.

ON GABOR'S ALTERNATE-SLAB MODEL

Gabor's alternate-slab model for calculating heat transfer between a wall and a moving packed or fluidized bed has been further developed using a physically more realistic numerical procedure. Two explicit techniques were examined. In one scheme, the new temperatures were calculated from the temperatures determined during the previous time interval. In the second scheme, the new temperatures were calculated using an average value of old and new temperatures calculated during the current time interval. The predictions of heat transfer rates for the refined alternate-slab model using old temperatures and those of the Mickley-Fairbanks model agree with each other at large values of t/dp 2 for the same gas gap thickness of 0.13dp. The second scheme, which has more realistic attributes, leads to a gas gap of O.16dp, which is in better agreement with the experimental data for air-glass and air-copper systems.

On Gabor's Alternate-Slab Model

Abstract Gabor's alternate-slab model for calculating heat transfer between a wall and a moving packed or fluidized bed has been further developed using a physically more realistic numerical procedure. Two explicit techniques were examined. In one scheme, the new temperatures were calculated from the temperatures determined during the previous time interval. In the second scheme, the new temperatures were calculated using an average value of old and new temperatures calculated during the current time interval. The predictions of heat transfer rates for the refined alternate-slab model using old temperatures and those of the Mickley-Fairbanks model agree with each other at large values of t/dp2 for the same gas gap thickness of 0.13dp. The second scheme, which has more realistic attributes, leads to a gas gap of 0.16dp, which is in better agreement with the experimental data for air-glass and air-copper systems.

Laminar natural convection boundary layers on near-horizontal plates

The first order boundary-layer analysis for the laminar natural convection flow over a semi-infinite horizontal or near-horizontal heated plate has been extended to include variable properties effects. Detailed calculations are presented for air and water flows. It is shown further how these calculations can be extended to predict heat transfer rates on plates of rectangular planforms. Because of the lack of sufficiently detailed experimental data, a thorough comparison between the latter and the present calculations has not been possible. However, such comparisons as have been possible are encouraging and a significant improvement on previous theoretical predictions has been achieved. Although higher order corrections have not been discussed in detail, possible matching problems have been investigated briefly. Furthermore, it has been shown that stress work effects in the energy equation and the influence of pressure on density and temperature in the state relation are not only solely higher order effects but are of little significance at such orders.

Heat transfer rate predictions in condensation on droplets from air‐steam mixtures

AbstractThis paper describes simulations of the combined heat and mass transfer to single water droplets falling freely in air‐steam mixtures. The effects of droplet size, initial velocity, and concentration of steam in the mixture are discussed. The results are to be used in the design synthesis of steam dousing chambers in which sprays containing droplets whose diameters range as high as 6 mm. are used.

Heat transfer rate predictions in condensation on droplets from air‐steam mixtures

AbstractThis paper describes simulations of the combined heat and mass transfer to single water droplets falling freely in air‐steam mixtures. The effects of droplet size, initial velocity, and concentration of steam in the mixture are discussed. The results are to be used in the design synthesis of steam dousing chambers in which sprays containing droplets whose diameters range as high as 6 mm. are used.

Heat Transfer by Radiation and Other Transport Mechanisms : 3 rd Report, the Entrance Region of a Flow between Parallel Flat Plates with Simultaneous Radiation and Convection

Consideration is given to heat transfer for the problem of simultaneous development of velocity and temperature profiles in the case of a laminar flow in the entrance region between two flat plates. The duct consists of two diffuse, black, isothermal parallel surfaces separated by a finite distance. A medium filling the space between plates emits and absorbs thermal radiation. Numerical solutions are obtained for a slug flow (Pr=0), a flow of the inlet region (Pr=1) and a fully developed flow that was shown in the 2nd report, and show the influence of parameters such as the optical thickness, the ratio of conductive energy to radiative energy and Graetz number on the temperature variation across the duct and the heat transfer rates. The radiative heat transfer rate is scarcely affected but the convective one is much affected by flow patterns. An approximate method for the prediction of heat transfer rates in any flow patterns is shown and the results are in good agreement with the exact ones in the region of large optical thickness.

The Evaluation Of A New Model To Simulate Heat Transfer Enhancement In Ribbed Channels

The improvement of overall performance in power to mass ratio of modern gas turbines can greatly be contributed to the effective cooling of the turbine blades. Optimization of coolant performance is essential as the coolant flow introduces losses which need to be minimized. This paper introduces a new model to simulate heat transfer in ribbed channels. In the ribbed sections large variations in turbulence levels occure and the secondary flows associated with these ribs are responsible for significant local heat transfer variations. Furthermore, the ribs produce a complex flow structure so that there is continuing need for a better understanding of the flow physics in ribbed ducts. A finite difference simulation of the fluid flow and temperature distribution in the inlet section of the cooling channel of a typically cooled turbine blade is used to evaluate the new model. Various configurations of rib turbulators in the blade passages were simulated. In the case of the 45° angled ribs, there is a strong sideways secondary flow component which produces both increased overall heat transfer as well as asymmetrical local heat transfer enhancement. The results show the flow structure and heat transfer enhancement and correlate local heat transfer variations to secondary flow details. In general the predicted heat transfer rates compare well with measured data for 90° ribs, but further work needs to be done to accurately model angled ribs. NOMENCLATURE

The Numerical Modelling Of Heat Transfer InTurbine Blades

Advanced aircraft engines employ turbine entry temperatures so high that cooling of the turbine blades is crucial. This paper focuses on the ribs generally used to enhance the heat transfer in turbine blade cooling passages. Secondary flows associated with these ribs are responsible for significant local heat transfer variations. Furthermore, the ribs produce a complex flow structure so that there is continuing need for a better understanding of the flow physics in ribbed ducts. The flow in a channel with four consecutive ribs on two opposite walls and a blockage ratio equal to 0.063 were simulated for angles of 90° and 60° to the flow direction and a Reynolds number of 80000. An inlet condition approximating fully developed flow in a smooth duct were used. In the case of the 60° angled ribs, there is a strong sideways secondary flow component which produces both increased overall heat transfer as well as asymmetrical local heat transfer enhancement. The results show the pressure loss and heat transfer enhancement for both rib angles. In general the predicted heat transfer rates and pressure losses compared well with measured data in the stable area. NOMENCLATURE C^Cj Turbulence constants Ĉ E Turbulence constants k Turbulent kinetic energy p Pressure Pe Peclet number Pr Prandtl number q Heat flux Re Reynolds number St Stanton number v Velocity Vt Tangential velocity d Distance to the wall e Rate of dissipation p Density <7,,cTk Turbulence constants T Shear stress /A Viscosity INTRODUCTION Increased engine efficiency in modern aircraft engines requires a high turbine inlet temperature for a given pressure ratio. These high turbine inlet temperatures are only possible by proper cooling of the different turbine components. Of the components the rotor blades deserve special attention because they have to content with high Transactions on Engineering Sciences vol 5, © 1994 WIT Press, www.witpress.com, ISSN 1743-3533

A Study of Directional Radiation Properties of Specially Prepared V-Groove Cavities

Emission and absorption characteristics of a V-groove cavity are analyzed and the predicted results are compared with experimental measurements. The configuration considered consists of a flat black base with diffusely emitting, specularly reflecting side surfaces. The local directional emissivity is given as a function of emission angle, position, side wall emissivity, and cavity geometry. The results show that, with proper selection of the parameters, the cavity can be made a strong emitter and absorber for directions which are nearly normal to the cavity opening area, and a weak emitter and absorber for directions which correspond to large angles measured from the normal. A simple expression for cavity depth is suggested for closely approaching the optimum condition of maximum directionality of the cavity. Some predicted heat transfer rates for surfaces composed of many cavities are also presented. These results illustrate the potential value of employing specially prepared grooved surfaces in applications where control of radiant energy exchange is required.

Turbulent heat transfer in drag reducing fluids

AbstractAn analysis is presented which extends the analogy between energy and momentum transport for turbulent pipe flow of purely viscous fluids to include drag reducing, non‐Newtonian fluids. The correlation by Meyer is used to predict friction factor and sublayer thickness for the drag reducing fluids. The use of the friction factor correlation with the heat transfer analogy makes it possible to predict heat transfer rates from simple measurements of pressure drop and flow rate for the drag reducing fluids. Some recent experimental data for two effective drag reducing fluids and for water are compared with the predicted heat transfer rates, and the mean deviation in Nusselt number is found to be +8.5% for all of the data. The heat transfer analysis predicts a reduction in Nusselt number accompanying a reduction in friction factor for a given Reynolds number and for Prandtl numbers greater than 1.

Comparison of Some Approximate Methods for Calculating Re-Entry Ablation of a Subliming Material

calorimeter. This pa t t e rn was observed with other gases also, and it was a reproducible effect. I t is believed t h a t the heat transfer ra te in the thi rd section included a radiative contribution which was absent in the other two sections, and a radiat ive correction would, therefore, bring the observed and the predicted values closer together. I t m a y be concluded t h a t the turbulent pipe flow equation (Eq. [1]) can predict heat transfer rates correctly, within 15 to 20 per cent, in regions as close to the inlet as 7 to 10 diam for a geometry as shown in Fig. 1. Since the observed results represent average conditions over 2 diameters it is probable t h a t heat transfer rates considerably greater t h a n those measured in section 1 prevail locally in the ups t ream portion of this section.