Ramón L. Frederick

Heat transfer in square cavities with partially active vertical walls

Natural convection of air in square cavities with half-active and half-insulated vertical walls is numerically investigated for Rayleigh numbers of 103–107. This problem is related to applications in solar collection and cooling of electronic components. Five different relative positions of the active zones are considered. While circulation depends strongly on the total exit length downstream of the active zones, heat transfer depends less on this parameter. Significant conduction effects occur even at a Rayleigh number of 105. Expressions for average Nusselt number in the five situations and hints on how to generalize heat transfer results to intermediate cases are given.

Heat transfer in a square cavity with a conducting partition on its hot wall

Natural convection in a square cavity with a conducting partition on its hot wall and with perfectly conducting horizontal walls is numerically studied. The divider length and the divider-to-fluid thermal conductivity ratio (Rk) are varied. For low values of Rk, reductions in heat transfer relative to the undivided cavity are obtained, especially for Rayleigh numbers of 104–105. This reduction is more significant when long dividers are used.

On the transition from conduction to convection regime in a cubical enclosure with a partially heated wall

Abstract Steady-state laminar natural convection in a cubical enclosure with a cold vertical wall and a hot square sector on the opposite wall is numerically studied. The flow pattern consists of a single, symmetric circulation cell. The transition from conduction to convection regime ends at Ra=105, and is characterized by conduction suppression and slow development of convection. In the range of Ra=105–107, lateral velocities become very large, producing a highly mixed, thermally stratified three-dimensional flow, and the circulation cell undergoes cross-sectional changes. In the whole range of Ra, the Nusselt number is reduced compared to the case in which the full side wall is hot (test case).

Spatial and thermal features of three dimensional Rayleigh-Bénard convection

Abstract A full 3-D numerical study is reported on natural convection of air in six heated-from-below parallelepipedic enclosures. A supercritical Rayleigh number (Ra = 8 × 10 3 ) , was used in order to track the evolution of convective structures as a function of the enclosures aspect ratio ( A ) only. The six situations (1 ⩽ A ⩽ 5) exhibit a characteristic toroidal flow pattern which can be seen, at the lowest aspect ratio, as a unicellular structure which evolves to a multicellular combination of concentric roll-cells at higher aspect ratios. The transition from one convective structure to another has the features of a flow bifurcation controlled by A . The overall Nusselt number changed continuously when the aspect ratio was increased. At the lowest aspect ratios the change was significant, but further increments in A did not produce important variations in this parameter, in the range of aspect ratios studied.

On the aspect ratio for which the heat transfer in differentially heated cavities is maximum

Abstract A numerical study of two dimensional natural convection of air in rectangular, differentially heated cavities was conducted. For three Rayleigh numbers the aspect ratio S was varied. It was found that heat transfer between the active walls has a maximum at each Rayleigh number, which is located at values of S between 1 and 2. The aspect ratio for which the maximum heat transfer occurs is determined by the process of transition from a shallow cavity regime to a slender cavity regime.

Natural convection heat transfer in a cubical enclosure with two active sectors on one vertical wall

Natural convection in a cubical enclosure with active surfaces on the same wall is studied numerically over a wide range of Rayleigh number. Nusselt numbers exceed the ones for a side heated cavity at low Ra. The transition to a convective regime is delayed to a very high Ra. The flow patterns and temperature distribution are described and an expression for overall heat transfer is proposed.

Natural convection in central microcavities of vertical, finned enclosures of very high aspect ratios

Natural convection in the central microcavities of vertical, finned cavities of very high aspect ratios were numerically investigated using a spacewise periodical approach. When equally spaced fins are located on both active walls (problem b ), different circulation rates are found in two consecutive microcavities. The average Nusselt number in a region comprising two consecutive microcavities is higher than the one observed when the fins are attached to only one of the active walls (problem a ). The dependence of microcavity circulation rates and heat transfer on Rayleigh number, dimensionless fin length, and microcavity aspect ratio is discussed. The limits of applicability of the spacewise periodical approach are also outlined.

Air-cooling characteristics of simulated grape packages

Abstract Experimental simulation of the external forced convection on the outside of grape packages was performed. Average heat transfer coefficients for air flow around such containers were found to range from 8 to 13.4 W/(m 2 K). A physical description of the convective process was formulated on the basis of data obtained in three types of experiment. Expressions for the average heat transfer coefficient from single packages in air flow were proposed.

Semi analytic solution to the Cartesian Graetz problem : results for the entrance region

A solution to the Graetz problem based on a series expansion of the temperature field is reported. The range of Graetz numbers considered has been extended up to a value of 10 9 . The local values of Nu produced by the method are believed to be highly accurate. A correlating expression for Nu, which shows nearly uniform accuracy over the whole Gz range is proposed and compared with previously available correlations