Relative Heat Transfer Research Articles

Heat-transfer distribution for an impinging laminar flame jet to a flat plate

Impinging flame jets are widely used in applications where high heat-transfer rates are needed, for instance in the glass industry. During the heating process of glass products, internal thermal stresses develop in the material due to temperature gradients. In order to avoid excessive thermal gradients as well as overheating of the hot spots, it is important to know and control the temperature distribution inside a heated glass product. Therefore, it is advantageous to know the relation describing the convective heat–flux distribution at the heated side of a glass product. In a previous work, we presented a heat–flux relation applicable for the hot spot of the target [M.J. Remie, G. Särner, M.F.G. Cremers, A. Omrane, K.R.A.M. Schreel, M. Aldén, L.P.H. de Goey, Extended heat-transfer relation for an impinging laminar flame jet to a flat plate, Int. J. Heat Mass Transfer, in press]. In this paper, we present an extension of this relation, which is applicable for larger radial distances from the hot spot.

Extended heat-transfer relation for an impinging laminar flame jet to a flat plate

Many industrial applications use flame impingement to obtain high heat-transfer rates. An analytical expression for the convective part of the heat transfer of a flame jet to a plate is derived. Therefore, the flame jet is approximated by a hot inert jet. In contradiction with existing convective heat-transfer relations, our analytical solution is applicable not only for large distances between the jet and the plate, but also for close spacings. Multiplying the convective heat transfer by a factor which takes chemical recombination in the cold boundary layer into account, results in an expression for the heat flux from a flame jet to the hot spot of a heated plate. Numerical and experimental validation show good agreement.

An extended two-dimensional mathematical model of vertical ring furnaces

An extended two-dimensional (2-D+) mathematical model of vertical anode baking furnaces has been developed. The work was motivated by the fact that a previous 2-D model was unable to predict the nonuniform baking in the transverse direction,i.e., perpendicular to the longitudinal axis of the furnace. The modeling strategy based on dividing each section in four zones (underlid, pit, underpit, head wall and fire shaft zones) and introducing two symmetry planes in the exterior pits is explained. The basic heat-transfer relations used are also detailed. Selected results shown include draught and oxygen concentration profiles in the flue, gas and anode temperature distributions and fuel consumption in the back fire ramp. Simulation and experimental results are compared.

Condensation of organic binary mixtures flowing horizontally in a horizontal tube bundle

Both heat and mass transfer in the gas phase and heat transfer in the liquid phase are examined experimentally for film condensation of organic binary mixtures such as ethanol-water and methanol-water. Experimental results on the average heat flux, vapor-liquid interface temperature and liquid-phase Nusselt number are compared with analytical solutions based on stagnant film theory and heat-transfer relationships for film condensation from a pure vapor. Experimental heat transfer results agree well with the analytical solutions, except that the experimental liquid-phase Nusselt numbers under conditions of low mass fraction of water are considerably higher than predicted by the analytical solutions. This high value of the liquid-phase Nusselt number is considered to be caused by dropwise condensation in the liquid phase. However, its effect on the tube bundle is not so remarkable compared with that in gravity-controlled condensation on a vertical surface. This is considered to be caused by the condensate inundation effect. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25(6): 342–361, 1996

Heat-transfer relations of avian nestlings

1. 1.|To determine the thermoregulatory prowess of altricial nestlings, we conducted both equilibrium and transient analyses of white-crowned sparrow nestings, a representative fringillid. 2. 2.|For an individual nestling at thermal equilibrium, feather development is the major factor reducing heat loss after 2 days of age; tissue- and boundary-layer resistances are of minor importance. 3. 3.|The nest substantially reduces wind speeds near the nestlings. Heat transfer through the nest material is of only moderate importance. Evaporation also appears to be a small proportion of total heat loss during hypothermia in natural environments. 4. 4.|Net long-wave radiant exchange is also minor, but short-wave radiation is potentially a major component of the nestling's energy budget, approaching the magnitude of maximal metabolic heat production. 5. 5.|When nestlings cool, their body mass and metabolic rate are also major importance in determining the rate of cooling, and (for metabolism) the equilibrium temperature as well. 6. 6.|The huddling together of nestlings is perhaps the single most important factor affecting heat transfer. 7. 7.|An older brood actually has more insulation than does an adult in the same microclimate.

Forced-Convection Heat Transfer from Irregular Melting Wavy Boundaries

The problem of turbulent flow past freezing or melting ice is formulated for the case of random boundary waves, with particular attention to the distribution of heat-transfer rate along wavy surfaces. Relations are obtained for the celerity and unsteady spectrum of the boundary waves. The results of 21 laboratory experiments are reported and used to establish the values of certain quantities appearing in the heat-transfer relation adopted. The velocity Reynolds number of the phase shift between the boundary waves and heat-transfer variation is found to have a nearly constant value of 4 × 104.

Observations of the gas-flow characteristics of low-current DC arcs

Previous work has shown that a low-current DC arc in an axial gas flow is analogous to a heated solid rod surrounded by a gas stream which removes heat from the surface. The heat transfer from the arc core is then described by the well-known Nusselt heat-transfer relationship for this situation. The present work tests the effect of different gases and electrode configurations. The relationship is valid for the non-reacting gases examined, and the different electrical characteristics obtained for each gas are accounted for simply by the different thermal/physical properties of the gases. The model enables such an arc to be characterized by measurements of electric field, current and gas-flow velocity. The arc characteristics for reacting gases such as propane/air mixtures are strongly affected by other factors such as electrode arrangement.Inferences about the gas flow in the arc system drawn from this model have led to the construction of a small double-annulus device that enables a gas fed to the inner annulus to be heated by the arc while being shielded from the surrounding atmosphere by an inert gas flow. It is suggested that this device may find applications in low-current-plasma chemistry.

Laminar Free Convection from a Rotating Radial Plate

The paper presents a theoretical analysis of conduction through, and free convection from, a radial plate rotating in a synchronous environment of air. The plate resembles a tapered, radially protruding fin heated at the root. Ordering of the governing equations reveals three controlling parameters, under the condition of steady high-speed rotation. Numerical solutions to the combined conduction–convection problem reveal the effect of the parameters on the velocity and temperature profiles, the overall heat-transfer relation, and the fin effectiveness.

Compressible Turbulent Boundary-Layer Heat Transfer to Rough Surfaces in Pressure Gradient

The velocity defect law and the law of wall for nonadiabatic compressible turbulent boundary layers on rough wall are presented in terms of the generalized velocities suggested by Van Driest. It is possible to correlate measured boundary-layer profiles, for a range of Mach number from 0 to 5 and wall temperature ratio TJTaw from 1.0 to 0.5, on the universal curves. From these correlations, the skin friction law for rough surfaces under various Mach number, heat-transfer rate and pressure gradients is generated. The corresponding heat-transfer relation is derived from the established incompressible semiempirical correlations based on the model that roughness effects on the mixing process is restricted to a relatively small region near the wall. The resulting equations are combined with the momentum integral equation and an auxiliary equation to provide a calculation method for predicting the development of the compressible turbulent boundary layer over rough surfaces in pressure gradient. The analysis is compared with a series of heat-transfer measurements on roughened hemispheres at Mach number 5 and 6. The agreement in general is good.

Bulk transfer coefficient for heat transfer

A bulk heat-transfer relation applicable to unstable conditions is proposed. The relation incorporates the assumptions that heat transfer near the surface is by molecular conduction, and the bulk transfer relation does not depend explicitly on the surface roughness characteristics, provided that roughness elements are not large in comparison with the thickness of the thermal boundary layer. The predicted heat flux compares favorably with heat-transfer rates determined in the Great Plains field program and with measurements made by J. E. Vehrenkamp over a desert dry lake under conditions of extreme upward heat flux. The limiting case of very small surface stress is discussed, and the proposed formulation in this limit is compared with the heat-transfer relation observed in laboratory experiments on parallel-plate convection.

Heat transfer by the induced flow about a rotating cone of non-uniform surface temperature

The present paper is concerned with the heat-transfer characteristics by the induced flow, laminar or turbulent, about a rotating cone with non-uniform surface-temperature distributions. Consideration is first given to the theoretical calculation based on various existing theories. Discussed in details are the two cases: the power-law and the step-change surface-temperature distributions. The case of step-change distribution is further studied experimentally through the naphthalene-sublimation technique. The step-change surface temperature is simulated by a step-change surface sublimation. Measurements of the sublimation rate from rotating cones of various vertex angles are made in both the laminar and turbulent flow regions. They are then compared with the predicted mass-transfer results obtained from the analytically established heat-transfer relation through the heat and mass-transfer analogy argument.

Particle drag and heat transfer in rocket nozzles

An empirical expression for the drag coefficient of a spherical particle in flow regimes such as occur in solid propellant rocket exhausts is presented. In these flows, typical particle Mach numbers will be below 2, and particle Reynolds numbers will range from less than 10 to greater than 100. Also, available heat-transfer relationships are applied to the gasparticle nozzle flow case. The effects of these relationships on computed particle velocities and temperatures are shown. In the cases considered, inertial and compressibility effects dominate for large particles and high chamber pressures, causing the thermal and velocity lags to be less than those predicted under a Stokes flow assumption. However, for small particles and low chamber pressures, rarefaction effects dominate, and the Stokes flow assumption leads to low estimates of particle lag.

Instabilities in the Flow of a Boiling Liquid

The time-varying flow of a boiling liquid through a series of ducts and heaters is considered for the case where the flow rate is controlled by a downstream orifice. It is shown that oscillatory flow will exist for a variety of configurations, provided that the density ratio across the system exceeds a critical value which depends on the geometry and the heat-transfer relationship.

Heat transfer by laminar forced flow against a non-isothermal rotating disk

Theoretical consideration is given to the temperature distributions and heat-transfer results in laminar forced flow against a non-isothermal rotating disk. The surface temperature of the disk is assumed to vary according to a power law with the radial distance. Numerical solutions of the boundary-layer equations are presented to indicate the effects of the forced flow, of the non-uniform surface temperature and of the Prandtl number. Asymptotic heat-transfer relations for large and small Prandtl numbers are given.

The Effect of Condensate Reheat on Mean Temperature Difference in Feedwater Heater Subcooling Zones

Abstract Heat leakage through the subcooling zone wall in combination feedwater heaters tends to defeat one purpose of the zone; namely, to subcool the condensate. Equations are presented for the determination of the mean temperature difference, feedwater and condensate-outlet temperatures, and minimum subcooling terminal approach under such conditions. These are given in terms of parameter K = (UsAz/UsAs), the relative heat-transfer capability of the zone envelope as compared to the tube surface enclosed.

Surface heat-transfer coefficients of salient poles in a blast-cooled alternator

THERE are few data available on which the calculation of heat-transfer coefficients of blast-cooled rotating machine structures and adjacent stationary internal component surfaces can be based. The application of fundamental heat-transfer relationships, derived from stationary simple configurations, is impractical because of the significant effects of rotation and their variations with density of the cooling air, which may occur in machine passages. From temperature measurements on actual machines, heat-transfer coefficients cannot be derived reliably because of the usual difficulties in determining the heat-flow distribution among the various surfaces of a component whose heat dissipation may be known.

Convective Heat Transfer for Mixed, Free, and Forced Flow Through Tubes

Abstract This paper presents current knowledge on heat transfer caused by simultaneously occurring free and forced convection in tubes. It discusses the results of experiments that have been conducted by the NACA on mixed, free, and forced-convection heat transfer in turbulent flow through a vertical tube with a length-to-diameter ratio of 5. In these studies, forced air flow was directed parallel or opposite to the flow direction which free-convection flow alone would have under the same temperature conditions. Results of experiments made by other investigators, when analyzed in the same way as the NACA experiments, make it possible to generalize and extend the heat-transfer relations established. A calculation by R. C. Martinelli and L. M. K. Boelter for laminar mixed, free, and forced flow in parallel direction, and its extension to flow in opposite direction furnish information on heat transfer in the laminar-flow range.

Aerodynamic Heating and Convective Heat Transfer—Summary of Literature Survey

Abstract The analytical and experimental information appearing in the literature for convective heat transfer at high velocities is summarized. Particular emphasis is given to heat transfer for submerged flow over flat plates, wedges, cones, and cylinders with a single reference to flow in pipes. The evidence presented supports, as a fundamental heat-transfer relation, the modified Newton’s law of cooling q=hA[tw−(to+ruo2/2gJcp)] where the term ruo2/2gJcp) accounts for the influence of frictional dissipation and is readily evaluated when the recovery factor r is known. A second and very important result is the evidence given to show that the heat-transfer coefficient h, is independent of the magnitude of the frictional-dissipation term. Data are presented for the recovery factor and heat-transfer coefficient for high-speed subsonic- and supersonic-flow velocities. It should be noted, however, that heat transfer by radiation is not considered here as it would be treated in the usual manner for heat transfer without frictional dissipation.

Heat Transfer From a Baffled-Finned Cylinder to Air

Abstract Air-cooled aircraft-engine cylinders utilize extensive finned surface to permit maintenance of proper metal temperatures with high rates of heat transfer occurring at high engine power outputs. The process of heat transfer to the flowing air stream differs from that of usual finned tubes not only in size of the cylinder but also because baffles are utilized which force all the air to pass through the fin spaces. Because of this latter difference, data on finned tubes in general do not apply. Therefore, special tests have been carried out to establish the mechanism of heat transfer in this case and to provide a generalization of the data useful for design. The work was carried out at the University of Delaware in conjunction with a Pratt & Whitney Aircraft heat-transfer research program. The experiments were conducted on a typical section of an aluminum finned cylinder barrel. The cylinder was heated by an internal electrical heater. Temperatures were measured not only of the entrance and exit air stream but also at eight points, equally spaced circumferentially, in the base metal. The results were interpreted by comparison with heat-transfer relations for flow inside conduits. The maximum heat-transfer rates were in close agreement with these relations although the average rates were lower owing to the decreased cooling-air velocities over the front and rear fin surfaces outside of the baffled portion of the cylinder. The over-all pressure drop is shown to be about double the estimated frictional resistance; the inlet and exit losses provide an explanation.