Showing papers in "Journal of Heat Transfer-transactions of The Asme in 1980"

Periodic Streamwise Variations of Heat Transfer Coefficients for Inline and Staggered Arrays of Circular Jets with Crossflow of Spent Air

Abstract: Heat transfer characteristics were measured for inline and staggered arrays of circular jets impinging on a surface parallel to the jet orifice plate. The impinging flow was constrained to exit in a single direction along the channel formed by the jet plate and the heat transfer surface. In this configuration the air discharged from upstream transverse rows of jet holes imposes a crossflow of increasing magnitude on the succeeding downstream jet rows. Streamwise heat transfer coefficient profiles were determined for a streamwise resolution of one-third the streamwise hole spacing, utilizing a specially constructed test surface.

Vapor/Liquid Interaction and Entrainment in Falling Film Evaporators

Abstract: The problem of vapor/liquid interaction and entrainment in falling film evaporators is analyzed. Attention is focused primarily on horizontal tube falling film evaporators where liquid falls from one tube to the next in either a droplet or column mode. A criterion is presented for the onset of column formation, and equations are derived for the deflection of droplets and columns due to vapor crossflow. Based on an experimental study of drop detachment and breakup, a correlation is established for the resulting droplet sizes. For high vapor crossflow velocities, a criterion is presented for the inception of liquid entrainment by a process known as stripping. Based on the foregoing models, conditions are defined under which vapor/liquid interaction and entrainment are important for falling film evaporators.

Long-Time Solutions to Heat-Conduction Transients with Time-Dependent Inputs

Abstract: An economical method for obtaining long-time solutions to one, two, or three-dimensional heat-conduction transients with time-dependent forcing functions is presented. The conduction problem is spatially discretized by finite differences or by finite elements to obtain a system of first-order ordinary differential equations. The time-dependent input functions are each approximated by continuous, piecewise-linear functions each having the same uniform time interval. A set of response coefficients is generated by which a long-time solution can be carried out with a considerably lower cost than for conventional methods. The method has applications in problems of air-conditioning calculations for buildings and in the development of load models for solar energy studies.

An Experimental Study of Falling Liquid Film Breakdown on a Horizontal Cylinder During Heat Transfer

Abstract: In this study, an experimental investigation was conducted with subcooled water film flowing over an electrically heated horizontal cylinder. The combinations of film flow rate and heat flux at which film breakdown occurs (i.e., dry patches appear on the surface) were determined. At the conditions prior to dry patch formation, the heat transfer coefficient was determined as well. The results showed that the heat flux needed to cause a dry patch increases with film flow rate. Also, prior to dry patch formation, the heat transfer coefficient increases with film flow rate. The effects of the tube spacing and the liquid film inlet temperature on the breakdown heat flux and heat transfer coefficient were also studied.

Measurement of Vapor Superheat in Post-Critical-Heat-Flux Boiling

Abstract: A differentially-aspirated superheat probe was developed to measure vapor temperatures in post-critical-heat-flux, dispersed-flow boiling. Measurements obtained for water, at low-to-moderate pressures and mass flow rates in a tube, indicated very significant nonequilibrium, with vapor superheats of several hundred degrees (/sup 0/C). Predictions of published correlations showed unsatisfactory agreement with the experimental results.

A Parametric Analysis of the Performance of Internally Finned Tubes for Heat Exchanger Application

Abstract: This paper presents a parametric analysis of the performance of internally finned tubes in turbulent forced convection for application to heat exchangers. The analysis compares the performance of an internally finned tube exchanger with that of an exchanger having internally smooth tubes. The calculations are performed for three important design cases.

Analysis of Laminar Heat Transfer in Internally Finned Tubes with Uniform Outside Wall Temperature

Abstract: An analysis is presented for fully developed laminar convective heat transfer in tubes with internal longitudinal fins and uniform outside wall temperature. The governing momentum and energy equations were solved numerically, with the influence of fin conductance accounted for by a single parameter. The distributions of fin temperature, fluid temperature and local heat flux (both at the fin and unfinned surfaces) are presented. These are shown to be strongly dependent on finned tube geometry and, in some cases, on the fin conductance parameter as well. Based on average heat transfer per unit area, the various fins proved more effective than the unfinned surfaces. Values for overall Nusselt number indicated significant heat transfer enhancement over smooth tube conditions.

Radiation properties for polydispersions: Application to coal

Abstract: The extinction and absorption coefficients and the asymmetry for factor for polydispersions of absorbing spherical particles are analyzed. The results are based upon Mie's theory for single spherical particles and particle size distribution found in practical systems. Dimensionless spectral radiation properties are shown to be independent of the explicit size distribution and functions only of the average radii and the index of refraction. The Planck and Rosseland mean coefficients are also presented and the dependence on temperature is explicitly denoted for a large practical temperature range. The results for coal with optical properties which are wavelength dependent indicate the usefulness of the dimensionless and mean properties.

Effects of Nonuniform Thermal Gradient and Adiabatic Boundaries on Convection in Porous Media

Abstract: The effects of different combinations of thermally insulated boundaries and nonuniform thermal gradient caused by either sudden heating or cooling at the boundaries or by distributed heat sources on convective stability in a fluid saturated porous medium are investigated using linear theory by considering the Brinkman model. In the case of sudden heating or cooling, solutions are obtained using single-term Galerkin expansion and attention is focused on the situation where the critical Rayleigh number is less than that for uniform temperature gradient and the convection is not maintained. Numerical values are obtained for various basic temperature profiles and some general conclusions about their destabilizing effects are presented. In particular, it is shown that the results of viscous fluid (σ = 0) and the usual Darcy porous medium (σ → ∞ ) emerge from our analysis as special cases. In the case of convection caused by heat source, since the effect of heat source is not brought out by the single-term Galerkin expansion, the critical internal Rayleigh number is determined using higher order expansion by specifying the external Rayleigh number. It is shown that, for values of σ 2 ≥ 2.45 × 10 5 , the different combinations of bounding surfaces give almost the same Rayleigh number and an explanation, following Lapwood, for this surprising behavior is given. It is found that the heat source's effect on convection decreases for wave numbers up to the value 2.2 and drops suddenly around the critical value of 2.4 and then increases up to 2.5.

Natural Convection on Both Sides of a Vertical Wall Separating Fluids at Different Temperatures

Abstract: This paper describes an analytical study of laminar natural convection on both sides of a vertical conducting wall of finite height separating two semi-infinite fluid reservoirs of different temperatures. The countercurrent boundary layer flow formed on the two sides is illustrated via representative streamlines, temperature and heat flux distributions. The net heat transfer between reservoirs is reported for the general case in which the wall thermal resistance is not negligible relative to the overall reservoir-to-reservoir thermal resistance.

A numerical investigation of thermal convection in a heat-generating fluid layer

Abstract: Finite difference solutions of the equations governing thermal convection driven by uniform volumetric energy sources are presented for two-dimensional flows in a rectangular domain The boundary conditions are a rigid (ie, zero slip), zero heat-flux lower surface, rigid adiabatic sides, and either a rigid or free (ie, zero shear) isothermal upper surface Computations are carried out for Prandtl numbers from 005 to 20 and Rayleigh numbers from 5 x 10 to the 4th to 5 x 10 to the 8th Nusselt numbers and average temperature profiles within the layer are in good agreement with experimental data for rigid-rigid boundaries For rigid-free boundaries, Nusselt numbers are larger than in the former case The structure of the flow and temperature fields in both cases is dominated by rolls, except at larger Rayleigh numbers where large-scale eddy transport occurs Generally, low velocity upflows over broad regions of the layer are balanced by higher velocity downflows when the flow exhibits a cellular structure The hydrodynamic constraint at the upper surface and the Prandtl number are found to influence only the detailed nature of flow and temperature fields No truly steady velocity and temperature fields are found despite the fact that average Nusselt numbers reach steady values