Showing papers in "Advances in heat transfer in 1990"

Thermal Control of Electronic Equipment and Devices

Abstract: Publisher Summary Thermal control of electronic components has one principal objective, to maintain relatively constant component temperature equal to or below the manufacturer's maximum specified service temperature, typically between 85 and 100°C. It is noted that even a single component operating 10°C beyond this temperature can reduce the reliability of certain systems by as much as 50%. Therefore, it is important for the new thermal control schemes to be capable of eliminating hot spots within the electronic devices, removing heat from these devices and dissipating this heat to the surrounding environment. Several strategies have developed over the years for controlling and removing the heat generated in multichip modules, which include advanced air-cooling schemes, direct cooling, and miniature thermosyphons or free-falling liquid films. The chapter summarizes analytical, numerical, and experimental work in literature, in order to facilitate the improvement of existing schemes and provide a basis for the development of new ones. The chapter focuses on investigations performed over the past decade and includes information on the thermal control of semiconductor devices, modules, and total systems.

Two-Phase Slug Flow

Abstract: Publisher Summary Slug flow is a highly complex type of flow with an unsteady nature The prediction of pressure drop, heat, and mass transfer for such flow is often considered a difficult task The chapter deals with steady slug flow The chapter introduces several options of modeling the hydrodynamic parameters and pressure drop using a unified approach that is applicable for the vertical, horizontal, and inclined cases The chapter also reviews transient phenomena in slug flow by illustrating an example of severe slugging in a pipeline-riser system It is noted that heat transfer in slug flow is of major importance for practical applications The treatment of the two-phase flow is usually considered as a two-phase mixture The severe slugging that consists of one riser and one pipeline is one of the simplest example of slug flow under nonsteady conditions The Boe criterion differentiates between steady and cyclic operations with two exceptions At high liquid flow rates, a steady flow can also exist within the severe slugging region predicted by the Boe criterion There is a region outside the Boe criterion that is in an unsteady state and leads to unsteady oscillations

Combined Heat and Mass Transfer by Natural Convection in a Porous Medium

Abstract: Publisher Summary The chapter describes the combined heat and mass-transfer natural convection mechanisms, which are considered an important subfield in contemporary heat and mass-transfer research. This subfield essentially brings together the studies concerned with the combined heat and mass-transfer or double-diffusive processes that are driven by buoyancy through porous media saturated with fluid. The density gradients that provide the driving buoyancy effect are induced by the combined effects of temperature and species concentration nonuniformities present in the porous medium. The chapter considers the phenomena of convection through fluid-saturated porous media generally in terms of volume-averaged quantities. There are four conservation principles considered in the study of convection with more than one buoyancy effect. These include conservation of mass, energy, species, and momentum. Heat and mass transfer in the vertical direction and in horizontal direction are discussed in detail. Another category of studies of combined buoyancy effects in porous media deals with the local fields around buried sources of heat and mass. The recent work in this field focuses on the multilayer structure of flows of the boundary-layer or concentrated-source type.

Nucleate Boiling Heat Transfer and Its Augmentation

Abstract: Publisher Summary Nucleate boiling is a complicated phenomenon accompanied with a phase change from liquid to vapor. The nucleate boiling heat transfer is one of the most important modes of heat exchange occurring in several constituent devices of thermal plant. The augmentation of nucleate boiling heat transfer significantly contributes towards the efficient use of the thermal energy. The chapter describes a correlation method of nucleate boiling heat transfer by considering the factors that affect the heat-transfer processes. It presents the effect of surface configuration on the nucleate boiling along with the nucleate boiling heat transfer in a narrow space and in a liquid film. The chapter also illustrates potential measures for augmentation of nucleate boiling heat transfer and some results obtained. As the nucleate boiling heat transfer is greatly influenced by surface conditions, the unified rule of the nucleation factor for several surface conditions, which is similar to the emissivity encountered in the heat transfer by thermal radiation, needs to be developed in the future.

Unified Regenerator Theory and Reexamination of the Unidirectional Regenerator Performance

Abstract: Publisher Summary It is noted that in both fixed-bed cyclic thermal regenerators and rotary-matrix exchanger, the sole mechanism of heat transfer between the flowing gases and the regenerator matrix is assumed to be forced convection, and this results in two coupled first-order partial differential equations describing the transfer in each of the two periods of operation The number of variables in each system is reduced by defining dimensionless groups and normalized temperatures leading to the development of separate theories for each of the two regenerator systems—fixed bed and rotaryThe fixed-bed system is represented by the dimensionless length Λ and the dimensionless period Π of each period The ratio of the dimensionless period to the dimensionless length is referred to as utilization factor A regenerator reaches cyclic equilibrium when the heat transferred to the matrix during the flow of the hot gas stream is equal to the heat released from the matrix during the flow of the cold gas stream

Applicability of Solutions for Convection in Potential Flow

Abstract: Publisher Summary The chapter aims to review, correct, and then assemble in a systematic form the solutions derived by Boussinesq and subsequent investigators for convection in potential flow. The purpose of the chapter is also to obtain solutions for convection in potential flow for additional important geometries and boundary conditions. The chapter then identifies the errors in these solutions resulting from several idealizations, and thereby determines their limits of applicability. The chapter also analyzes the applicability of solutions for convection in potential flow for moderately low Prandtl number fluids and the applicability of solutions for convection in potential flow for rising bubbles. Further, the chapter discusses the utility of solutions for convection in potential flow in the development of correlating equations for all Prandtl numbers. It is mentioned that the method of Boussinesq provides a simple, systematic procedure for the derivation of solutions for convection from bodies immersed in an unbounded potential flow. This method is utilized to compile a set of solutions for several other common shapes.