Showing papers in "Journal of Heat Transfer-transactions of The Asme in 1979"

Heat Transfer Characteristics for Inline and Staggered Arrays of Circular Jets with Crossflow of Spent Air

Abstract: Heat transfer characteristics were measured for two dimensional arrays of 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. Both mean Nusselt numbers and streamwise Nusselt number profiles are presented as a function of Reynolds number and geometric parameters. These are the streamwise and transverse hole spacings ranging from 5 to 10 and 4 to 8 jet orifice diameters, respectively; the channel height ranging from 1 to 6 diameters; and the hole pattern which includes both inline and staggered arrays. The results show that significant periodic variations occur in the streamwise Nusselt number profiles, persisting downstream for at least ten rows of jet holes. Channel height can have a significant effect on the chordwise profiles, smoothed across the periodic variations. For the smaller channel heights, Nusselt numbers first decrease and then increase downstream. Where significant differences exist, inline hole patterns provide better heat transfer than staggered ones. These and other effects of the geometric parameters are presented and discussed.

Techniques for reducing thermal conduction and natural convection heat losses in annular receiver geometries

Abstract: An effective device for the collection of solar energy which has received widespread attention is the so called parabolic-cylindrical solar collector. In this design a circular receiver tube, with a suitable selective coating, is enclosed by a concentric glass envelope and situated along the focal line of a parabolic trough reflector. The heat transfer processes which occur in the annular space between the receiver tube and the glass envelope are important in determining the overall heat loss from the receiver tube. In typical high temperature receiver tube designs the rate of energy loss by combined thermal conduction and natural convection is of the same order of magnitude as that due to thermal radiation, and can amount to approximately 6 percent of the total rate at which energy is absorbed by the solar collector. The elimination of conduction and natural convection losses can significantly improve the performance of a large collector field. Several techniques useful for the reduction of energy loss by thermal conduction and natural convection are considered. The receiver configuration chosen for study is typical of those used in the Solar Total Energy System at Sandia Laboratories. The receiver tube has a ''black chrome'' selective coating and is 2.54 more » cm in outside diameter. The inside diameter of the glass envelope is approximately 4.4 cm. Typical operating temperatures of the receiver tube and glass envelope are approximately 300/sup 0/C and 100/sup 0/C, respectively. « less

Convective Heat Transfer in Porous Media

Abstract: Convective heating or cooling of granular solids or porous media is of interest in the design of thermal energy storage systems The solutions to the energy initial boundary value problems governing convective heat transfer between a fixed bed of granular solids and a steady flow of heating or cooling fluid are presented The storage system is considered to be initially in thermal equilibrium at a uniform temperature, a step change in the inlet temperature of the working fluid is imposed, and the thermal response of the system predicted The results are valid for gases and liquids when the temperature gradient in the solid material is small and axial conduction effect is negligible in comparison with the convective heat transfer Unlike the previously available solutions to this problem, the results presented are in closed form This greatly simplifies evaluation and design of thermal energy storage systems of this general type

Low-Velocity Heat Transfer to a Flat Plate in The Presence of a Corona Discharge in Air

Abstract: A study was conducted to determine the enhancement in convection heat transfer that could be achieved using the corona wind over a range of stream velocities. A heated flat plate mounted in a flow channel was placed in a Mach-Zehnder interferometer. Corona wires were placed above the active plate surface. Data were taken over a range of stream velocities. The results showed the expected large increase in heat transfer at low velocities and that this gain in heat transfer decreased to zero at high stream velocities.

Two-Phase Flow on the Shell-Side of a Segmentally Baffled Shell-and-Tube Heat Exchanger

Abstract: This paper reviews work carried out at the National Engineering Laboratory, UK., related to pressure drop flow patterns and phase distribution on the shell-side of segmentally baffled shell-and-tube heat exchangers. The experimental work reported was carried out using air/water mixtures in model exchangers of rectangular cross section with tube nests containing approximately 40 tubes. Data were obtained on crossflow pressure drop and on the pressure drop attributable to the windows. In certain configurations the void fraction and flow pattern maps were obtained. The geometric conditions examined related to configurations appropriate to operation as condensers and boilers. Correlations for pressure drop and void fraction were developed and flow pattern maps obtained.

Mixed Convection on Inclined Surfaces

Abstract: An analysis is performed to study the effects of buoyancy force on the heat transfer characteristics of laminar forced convection flow over an inclined flat surface which is either maintained at a uniform temperature or subjected to a uniform heat flux. Numerical results are presented for Prandtl numbers of 0.7 and 7 over a wide range of values of the buoyancy force parameters, with the angle of inclination ranging from 0 to 90 deg from the vertical. In general, it is found that for both surface heating conditions, the local friction factor and the local Nusselt number increase with increasing buoyancy force for assisting flow and decrease with increasing buoyancy force for opposing flow. In addition, the effects of the buoyancy force on these two quantities are found to diminish as the angle of inclination increases. A comparison is also made of the results between the case of uniform wall temperature and the case of uniform surface heat flux.

An Evaporating Ethanol Meniscus—Part I: Experimental Studies

Abstract: The profile of an evaporating ethanol meniscus was measured as a function of the evaporative heat flux using interferometry. A measure of the evaporative heat flux was obtained using vapor deposited resistance thermometers. The meniscus profile was found to be stable and a function of the heat flux for the heat flux range of 0 – 1.36w/m of interline. These results were used in an analysis of capillary flow heat transfer in Part II.

Freezing Controlled by Natural Convection

Abstract: Experiments were performed for freezing under conditions where the liquid phase is either above or at the fusion temperature (i.e., superheated or nonsuperheated liquid). The liquid was housed in a cylindrical containment vessel whose surface was maintained at a uniform, time-invariant temperature during a data run, and the freezing occurred on a cooled vertical tube positioned along the axis of the vessel. The phase change medium was n-eicosane, a paraffin which freezes at about 36/sup 0/C (97/sup 0/F). In the presence of liquid superheating, the freezing process is drastically slowed and ultimately terminated by the natural convection in the liquid. The terminal size of the frozen layer an the time at which freezing terminates can be controlled by setting the temperature parameters which govern the intensity of the natural convection. The stronger the natural convection, the thinner the frozen layer an the shorter the freezing time. In the absence of liquid superheating, a cylindrical frozen layer grows continuously as predicted by theory, but the growth rate is higher than the predictions because of the presence of whisker-like dendrites on the freezing surface.

Turbulent Boundary Layer Heat Transfer on Curved Surfaces

Abstract: Heat transfer measurements for a turbulent boundary layer on a convex and concave, constant-temperature surface are presented. The heat transferred on the convex surface was found to be less than that for a flat surface, while the heat transferred to the boundary layer on the concave surface was greater. It was also found that the heat transferred on the convex surface could be determined by using an existing two-dimensional finite difference boundary layer program modified to take into account the effect of streamline curvature on the turbulent shear stress and heat flux, but that the heat transferred on the concave surface could not be calculated. The latter result is attributed to the transition from a two-dimensional flow to one which contained streamwise, Taylor-Gortler type vortices.

Forced Convection Heat Transfer on Heated Bottom Surface of a Cavity

Abstract: Experiments to measure the heat transfer characteristics for various cavities situated at a duct-wall were performed. Flow visualization, measurements of pressure and temperature distributions on the heated bottom surface of cavity were carried out. It was observed that the effects of main flow stream, reattachment of separated flow, and vortex flow in the cavity on heat transfer unexpectedly large. It was found that heat transfer did not always decrease monotonously with an increase of aspect (depth-width) ratio D/W, in the flow range of laminar to turbulent. Correlations between Num and Rew were made in laminar and turbulent heat transfer ranges.

Upper limit of CHF in the saturated forced convection boiling on a heated disk with a small impinging jet

Abstract: Critical heat flux (CHF) in the forced convection boiling on an open heated disk being supplied with saturated liquids through a small high-speed jet impinging at the center of disk is studied experimentally employing Refrigerant 12 at comparatively high pressures from 6.0 to 27.9 bars as well as water and Refrigerant 113 at atmospheric pressure. Generalized correlations of CHF are obtained for two characteristic regimes: V-regime where CHF is variable and I-regime where CHF is invariable for the change of jet velocity. Then, the boundaries of each regime are discussed clarifying the aspects for the lower limit of jet velocity capable of generating I-regime as well as the upper limit of CHF.

Natural convection heat transfer characteristics of flat plate enclosures

Abstract: Heat transfer by natural convection in rectangular enclosures has been experimentally studied using interferometric techniques. The effects of Grashof number, tilt angle, and aspect ratio on both the local and average heat transfer coefficients have been determined. The Grashof number range tested was 4 x 10/sup 3/ to 3.1 x 10/sup 5/, and the aspect ratio (ratio of enclosure length to plate spacing) varied between 9 and 36. The angles of tilt of the enclosure with respect to the horizontal were 45, 60, 75 and 90 deg. Correlations are developed for both local and average Nusselt number over the range of test variables. The effect of tilt angle is found to reduce the average heat transfer by about 18 percent from the value of 45 deg to that at 90 deg. No significant effect of aspect ratio over the range tested was found. A method for characterizing the flow regimes that is based on heat transfer mechanisms is proposed.